The most likely culprit may be that you do not have the transmitter properly seated in the camera's hot shoe. If you've insured this is not the case, along with making sure you do not have flash set to Disable in the camera's menu, then read on.
If your exposure settings - ISO, S (shutter time value), and A (aperture value) - are set to an exposure value for which there is already more than sufficient light in the scene based on your camera's meter reading, then the flash may not fire in TTL modes. When you are outdoors it is very possible that the use of flash is forcing the camera to use a shutter time no shorter (faster) the camera's flash sync speed of 1/200 unless you have Auto FP a/k/a High Speed Sync enabled in the camera's menu. Combined with your selected aperture and/or ISO settings this may leave no room for additional light from the flash without forcing the image to be overexposed. In such a case the camera will not fire the flash.
If you want to insure the camera will fire the flash you can do one or more of several things:
- Reduce the ISO setting if it is higher than the camera's base ISO 100
- Stop down the aperture if it is not already at the narrowest opening (highest f-number)
- Change the camera's metering mode from Matrix or Center Weighted modes that use the full frame to calculate exposure to Spot mode that only uses a small area in the center of the frame to calculate exposure so that the brighter parts of the scene do not unduly influence metering if you have a darker subject (realizing that the rest of the frame may be blown out in order to expose the dark subject more brightly)¹
- Enable Auto FP in the camera's menu to allow for High Speed Sync mode while realizing that the shorter the exposure time the more limited your maximum flash power will be
- Use a neutral density (ND) filter to attenuate the amount of light entering the lens so that your desired aperture and and exposure time will not result in overexposure at the camera's flash sync speed of 1/200 or longer exposure times
- Use manual flash power to set the desired output of the flash independent of the camera's metering routines while realizing that exposure times shorter (shutter "speeds" faster) than 1/200 will not allow the full sensor to be illuminated evenly by a single pulse of the flash
- Combine an ND filter, Auto FP, and manual flash power control to give you the most control over how you want the image to look
¹ Be aware that when using third party flash systems, using certain metering modes in TTL flash mode can sometimes give unpredictable and undesirable results. So while this may work better using Nikon flashes and triggers with Nikon cameras, it may not work as well, or at all, using reverse engineered Godox flashes with Nikon cameras.