Check the back of the magazine area just under the boltface - there should be a small plunger that lifts to hold the bolt when the mag follower is fully up (the gun is empty). Maybe this plunger is dirty, or damaged?
I use aftermarket 30-round mags in both my Yugo bullpup and my Norinco, and if I get just a bit of oil in the action, sometimes the last-round-holdback doesn't function.
I attribute the fact that you can manually get it to latch is because you aren't slamming the bolt back and then forward, as happens when you take a shot, and the system functions as it does; under pressure and very quickly!
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