Well, if I had the heads, inserts and a full compliment of precision machine shop inspection equipment, I could tell you what the problem is. Maybe the theaded shank on the Wasps is slightly undersized and lets them get past the boogered up part of the threads. Who knows.

Head on off to the hardware store and get an 8-32 UNC-2A tap and chase the threads in the inserts. Might be cheaper just to get new inserts though.