Without sample images and specific exposure information it's difficult to do anything but guess at what may be causing your issue.
Long exposures aren't normally a problem. Astrophotography images usually are noisy and that usually needs to be processed out.
Astrophotography usually tries to accentuate things in the sky that are normally hard to see ... "faint fuzzies". To bring these out, longer exposures are typically used ... but those come with side-effects. So there's a lot of post-processing work to manipulate the image in an effort to bring up the "faint fuzzies" well enough to make them visible while not over-blowing everything else. This is called "stretching" the image because if you think of the overall histogram of an image, you are trying to bring up the very faint stuff without also increasing the exposure on the stuff that is already bright.
That is to say... it may be that you actually have very good data and it simply needs to be processed and "stretched" to get the pleasing final result you want.
Here's an example:
I know it looks ugly and it's probably hard to see... but the above is actually good data. This is the Dumbbell Nebula (M27).
Most of what you are seeing in this "washed out" image ... is urban light pollution.
This is a 4 minute long exposure shot through an f/11 telescope that had a focal length just slightly above 3900mm. But the telescope is, unfortunately, located in an urban setting so there is a LOT of light pollution. This is what the image looks like straight-out-of-the-camera and no processing.
With a lot of images and a LOT of processing ... it gets turned into this:
It's hard to believe all that detail was hiding in such a muddy image. But this is what I mean by sometimes an ugly photo actually may be good useful data (but you'll have to do a lot of processing).
The better way to deal with light pollution is to take your camera far away from the city and try to find a nice dark-sky location to shoot (such as a dark-sky park). Depending on where you live ... getting away from urban light pollution may be difficult (I have to drive a couple of hours to get away from it).
Here are some samples I took of the lower region of Orion. THESE photos were not shot in an urban light-polluted location. These were shot in some of the very best skies on the planet. I was invited up into Science City ... the observatory complex atop Haleakala on the Hawaiian island of Maui. It was very dark and very clear. Photos straight out of the camera already looked good even without any processing at all.
I shot a series of different exposures because the the core of the main nebula (M42) is bright enough that it would be completely blown out in an exposure long enough to try to capture fainter areas of nebulosity and dust ... but a short exposure that doesn't blow out the core wont pick up any of the faint nebulosity or dust.
My strategy was to take a set of 2 minutes exposures, 1 minute exposures, 15 second exposures (except I accidentally set the intervalometer to 14 and didn't catch it ... but close enough), and 3 second exposures.
It is very easy to over-expose stars when trying to capture faint nebulosity. If that happens then the red and green and blue channels all saturate and you end up with "white" stars (most stars have some color cast and should appear slightly blue or yellow or orange, etc. but shouldn't just be "white") A lot of astrophotographers will take long exposures to capture nebulosity ... and then short exposures to capture the stars (without losing the color) and then merge them.
But long exposures in and of themselves don't necessarily just get washed out ... unless they are over-exposed. Seeing "noise" in an image is usually a sign of an under-exposed image that has had a lot of "gain" added to it (which increases the visibility of noise inherent in every image made by all cameras.)
The images here have not had any processing (other than converting the RAW into JPEG and reducing the size down considerably before uploading). There has been no adjustments for things like exposure, contrast, de-noising, etc. etc.
All images were shot with a Canon 60Da using a 135mm lens at f/2 and ISO 800. The 'a' suffix on 60Da means this is Canon's 'astrophotography' version of the camera (similar to Nikon having a D810a). Camera sensors are actually more sensitive to visible spectrum light than the human eye. An unmodified sensor can detect around 4-5x more 'reds' than the human eye. So a normal camera filters the visible spectrum to reduce transmission of some wavelengths so that the photos resemble what your eye sees. Astrophotography cameras use different filters (or sometimes just remove them entirely) so that the camera is more sensitive to as much light as it can get.
For this reason, these un-edited photos will appear more 'red' than you might expect. This is normal for an astrophotography color camera.
This is a 2 minute exposure:
Some things to notice in the above photo are that the core of the M42 nebula (right-most nebula in the image) is blown out from over-exposure. But you can see more nebulosity overall. If you look carefully there is a muddy background overall that appears clearer in some areas and less clear in others ... in a sort of blotchy way. That's not a camera problem.... that's real dust in space. Some astrophotographers will really try to bring that out to accentuate it in the final image.
Also notice the lower left corner shows the M78 nebula (barely visible) and the extreme corner has a touch of redness ... that's actually just catching a tiny bit of a feature called Barnard's Loop (that nearly encircles the entire constellation of Orion).
The scratchy lines that almost look like I shot this with film and scratched the negative (it's not film) are little micro-meteorites (every one of my longer exposure frames has them.... but they're all in different places so the 'integrated' final photo doesn't show those at all. They disappear because I used Winsorized Sigma Clipping as my integration algorithm.
This was 2 minutes at f/2. I've done other images and depending on the focal ratio I might go much longer. Some images are 5 minutes, some are 8 minutes, etc. (These are always on tracking mounts. You'd get obvious star-trails if the mount weren't tracking.)
There is noise in the images that get processed out as part of the integration process. That's normal and nothing to worry about and ... shooting shorter exposures typically results in the noise being more visible because you tend to have to boost the images to compensate for the shorter exposure... and that amplification ends up making the noise more apparent.
Each image contains some amount of "signal" (real data we want) and "noise" (which we don't want). We try to improve the "signal to noise ratio" (SNR) by using exposures which collect as much signal as we can get. You tend to get close to the same amount of noise in each image ... but noise is complicated because it isn't caused by just one thing. There are many kinds of noise. But usually when you take a longer exposure, the amount of signal grows faster than the noise ... so the longer image will usually have a better SNR.
This is a 1 minute exposure:
In this image the reddish glow in the extreme lower left corner is gone. Also the blotchy muddy background of dust is very difficult to see. But the nebula is smaller (not as much of the weak nebulosity shows up) and the blown out core is smaller (still blown out ... just not as badly).
This is a 14 second exposure:
In this image the red background is almost completely gone. The Horsehead and Flame nebulae are still visible... but just barely. The M42 nebula is considerably smaller and the blown out area of the core is also smaller ... but still technically blown out.
This is a 3 second exposure:
Finally here is the 3-second version. In this image, pretty much every bit of nebulosity except for the core of M42 is gone. I still technically would say this image is just slightly over-exposed but was good enough for my purposes (if I did this again ... I might take a 1-second or even a .5 second exposure).
In reality I took many images of each exposure duration and each 'set' of the same exposure duration were all stacked and integrated to create a single master exposure of that duration. Those four masters were then merged as an HDR to create the final image:
