"13thmonkey" said: "If it is on AF, and it is not able to focus, or achieve an adequate outcome it simply may choose to not take the shot."
I ended up browsing through this old thread, because I was having the same issue with a recently purchased used T7i (800D) unit. I discovered that the above was the actual problem. Canon has, apparently, designed their DSLRs to reject poorly taken photos… before they can happen. Lol. I was just trying to take a photo, quality not so important, while learning what all the buttons and settings on it were about, when I encountered the problem of the malfunctioning shutter button. When I adjusted the lighting in my photo (thank goodness I knew how!), the shutter button returned to it's former, fully-funtioning glory.
If you're having this problem, I would recommend adjusting your lighting… then try the shot again.
If you just want complete control over your "poorly taken" photos (lol), just put it on MF, and grab a book or take a class, and be done with it. All the best. - Erin (10/11/18, 4:20 p.m.)