Who did your conversion?
So you are using replacement springs in all of the mags? What happens if you use the factory UMP mag spring?Even out of the gun I can push down on the back of the shell and it won't pop back up like they will in the USC mags. It seems maybe the springs are not strong enough to straighten out the shells. It's just weird that it's happening on three different makes of magazines.
I did however get all three springs from hkparts.