Pistol is a Gen 4 G19. If I insert my loaded Glock brand 33 round mag and try to chamber a round, the action of the pistol will not close. It's not stuck on the slide lock but looks like it stuck partially open from the top round / mag. The magazine is seated properly prior to me doing this. If I simultaneously push up on the mag and pull and release the slide, a round will chamber. But when I then manually pull the slide back to eject the first round and then let it go to chamber the second, the action gets stuck open again. Regular size Glock brand magazines work just fine so I don't think it's a pistol issue. The 33 round mag is normally stored fully loaded. I don't really use it but just have it ready to go for home defense. It's worked without issue in the past. Of course I will be swapping it out until I get this issue resolved.

Any idea what the problem could be? Mag spring? I also wouldn't rule out "user error" as I could be overlooking something simple. Thanks!