I just went through something similar a few weeks ago with my ACC inserts. Some broadheads and field tips I had would seat fully, others wouldn' t. Turned out the insert threads were to blame, and a quick reaming with an 8/32 tap straightened things out. I got the tap at a hardware store for a few bucks.

When you say won' t thread, do you mean fully or at all? If they start to go, but then don' t, chances are you need to do the same thing, especially given the fact that both the field tips and broadheads worked in your other arrows...