Quoted:
I'm not Joe Expert, but when I was having a similar problem, it was suggested that I check the bolt stop for free movement. Mine would move, but it would bind, so a friend took out the bolt stop, and honed the sides lightly. From then on it moved freely, allowing the mag follower to push the bolt stop to the fully engaged position. Good luck.
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I'd agree totally if this was a new or nearly new gun; but he's saying that it's been used quite a bit, and just recently started acting up. So let me add the possibility that some sort of crap has gotten into the slot that the holdopen body rides in, and is causing the hang ups.
Either way, whether ir's a burr on the holdopen piece itself, or some junk in the slot, it is almost certainly the holdopen not fully rising to engage the front of the bolt. The only other possibility is that the mag springs have weakened to the point where they're not pushing the follower up firmly enough to fully engage the holdopen arm.
With the action open, work the holdopen up and down with your fingers. If you feel friction, then it's a burr or some crap in the slot. If it moves up and down freely, try a brand new mag, or replace the follower and spring in an old mag. If niether solution fixes the problem, get back to us for further consultation[:D]