I've found that a few makes of AK and G36 mag need a tiny bit filed/sanded off the underside of the tongue/mortice/lever/whajimmacallit, the bit which clicks past the sprung mag catch, to make it easier to lock.
Also a single drop of Abbey Silicone Oil into the BB feed hole of each mag makes all the difference in getting as many BB's in there as poss and, more importantly, having them feed out properly. Fill it with BB's to get the oil spread around inside - just release the sprung feed catch to let them all fly out into a plastic bag, then reload the same BB's and do it again a few times for each mag. Don't fire the oiled BB's or your hop rubber will not work properly until you've cleaned the oil off.