Originally Posted by Shooter45
The problem could be,as Marlin said, a magazine problem or a weak recoil spring. Short barreled 1911 style pistols need to have the recoil spring changed about every 2500 rounds.
Unfortunately I didn't ask to try another magazine.
To describe the problem more clearly, the last round in the magazine would chamber, but the slide would not return to full battery. It was a few milimeters out of battery, enough to prevent it from firing (and the safety could not be placed into the "on" position).
Now that I think about it, Shooter45, it does sound like a worn out recoil spring.