Intermittent issue where after I shoot around 50 rounds the bolt lock won't release consistently after loading a new mag and trying to chamber the first new round. Pull the bolt back but the plunger pin stays locked. Finally I push down mag follower, slide unlocks and cartridge loads and everything else works fine. I had polished up the plunger as I though the problem was it was binding in the hole but I'm starting to think my diagnosis might be wrong. Have shot around 400 rounds through it. No other issues like FTF or FTE. Suggestions of other areas where it may be binding?