Empty magazine will work, magazine with ammo won't go in

[Robert in NC]

Thanks to everyone for suggestions.

The mysterious problem, which I worked on for an hour and a half and not find a cause, has just as mysteriously solved itself. I took off the grips and still couldn't see anything wrong, and the problem persisted, so at a growing frustration level put the gun on the work bench and left it for several hours. When I decided to look at it again inexplicably everything worked fine.
Basically it didn't work, I left it on the work bench for several hours, and without doing anything else it did work.

While I'm glad the issue is solved I would have liked to know what the problem actually was and how it suddenly resolved itself. Operator error springs to mind, but it's not like inserting a magazine is a difficult operation.