If I get you right you are saying your Muzzy's won't screw into the insert all the way, correct? It's possible that some glue got into the threads in which case all you need to do is run an 8x32 tap in the insert to clean the threads. OR, maybe you got a bad batch of inserts.

Very seldom does anybody screw up the broadheads but it can happen. Compare the length of the threaded portion to your field points to see if they are too long.

If it's not too bad you might be able to just get a batch of nylon washers to put behind the heads.