Presuming they're actual M57 military surplus magazines and not recent aftermarket or converted CZ-52 mags, I'd say it's likely either the follower or the mag release.
One of the three magazines that I received with my two M57s had a ever-so-slightly bent follower that, upon firing the last shot would lock itself into the magazine well. The follower would actually protrude forward a smidgen when the last round was stripped out and chambered by the slide and make contact with the bottom of the feed ramp, preventing downward movement.
If your magazine exhibits these sticking issues with a loaded magazine inserted, then it's not your follower but probably your mag release itself (which could leave tell-tale drag mark on the magazine's body from the release notch on up). Being that both your magazines exhibit this problem, I would suspect the mag release to be the culprit.
If they do stick only when empty, then press down on the follower from the top with the slide locked back and hit the mag release. If it comes out without delay, there's your problem (the followers).
The M57 is a wonderful and robust pistol and in my opinion an improvement in terms of ergonomics over the TT-33. Once you resolve your magazine issue, I really believe you'll come to cherish the thing.