My Canon T3I on occasions will not take a picture. I will snap a shot then try again giving adequate time and it will fight me on taking the picture or just not release the shutter at all no matter what I do even though I as pressing the button. At times I have to give up on it all together and use something else to get the shot. I though battery voltage and that is not it brand new batteries in it. It is about 4 1/2 yrs old still original parts. I am hoping the shutter is not dying but it seems this may be the issue. I do not currently know the count on it but it has to be up near 80k by now. I am not a professional but I do enjoy photography and this thing comes everywhere with me. Any opinions?
Do you have focus priority or shutter release priority? Do you have a filter on the lens that may effect focus? Is there a pattern to when this happens? One pattern I have seen is taking a camera from an air conditioned vehicle into a hot summer day. This can create a slight condensation on the filter which prevents the autofocus from locking in. If you have focus priority this can be an issue.