How long are the threads on the field points as compared to the threads on the Muzzy's? It might be that the b-heads have longer threads and they are either getting caught up in some excess glue that is further down the insert...or maybe the insert isn't deep enough for the b-head. I kind of doubt the second choice but it could be. If you can, pull one of the inserts out of the arrow so you can look at it from all angles. That will prolly be the best way to diagnose the problem.