I have a Bushmaster CAR15. Old school style. Preban. In great shape though. Im having a problem with it though. With the bolt closed….if I try to insert a loaded magazine into the magwell, it will not "catch" and retain in the rifle….it will drop free to the ground. If I have the bolt rearward, the magazines lock up normally. Ive tried two different PMAGs and it happens with both of them. Any ideas of why the magazine won't engage with the bolt closed? Ive tried slapping the mag from the bottom but no joy. I will be taking a carbine course in a few weeks, and I am sure I will need to feed the rifle loaded magazines with the bolt closed