Forerunner 945 back button keeps getting stuck and unable to use.

I haven't had your problem, but it reminds me of my camera Olympus Tough-4 where the zoom-button was glitchly - it would get stuck when used. My guess is that I had used it in saltwater and hadn't rinsed it properly. After having submerged the camera in fresh water, I started to press the knob to the right and the left, zooming and unzooming. After a while it seemed to loosen up whatever that made it stay in position previously, and has since worked well. Maybe not helpful here, but submerging the watch in fresh water and trying to press the button maybe will make some difference - or not.

I've had the same issue on my watch. It's important to remember to clean the watch on regular basis. sweat, salt etc will affect the buttons after a while. Garmin has a cleaning guide somewhere. I usually put it in freshwater for a couple of hours and use a soft brush around the buttons.