[Thor]

I have seen this before, usually either a weak hold open spring and the hold open is probably cycling from firing vibration during the firing/loading sequence or it gets caught by the frame if it sticks a bit. The other cause was magazines that actually lift the hold open with one round remaining. Rick is right, take the top off the frame and put a loaded/and unloaded mag in the frame and see how the hold open moves, sticks (or not). A lot of the time you can tell quite a bit with just an empty mag in there and push the hold open down with your thumb, does it pop up smoothly? Good luck!