Springfield G.I., an affordable 1911

[Jack]

Hmmmmm...
Take a close look at the relationship between the slide stop and the magazine- I wonder if you have some binding there.
My 1911 isn't a Springfield, but Chip McCormick mags always bind on the slide stop on my pistol. If I take a file to the magazine and cut the slide stop opening on the magazine back a bit, they work perfectly.
If you have the slide stop binding on the magazine, that would keep it from falling free also. Of course, trying another brand of magazine might be worth a try, too.
Wilson and Brown are 2 brands of magazine that work fine in my gun- and so do the McCormicks once I tune them with a file.