I changed our stove pipe from 6" down to double walled 3 Simons duravent. I can not get the pipe onto my Harmon P61A. It only goes on about 2 inches. I think the tabs are keeping it from going on further.(The tabs lock into the next piece, but since the pellet stove has a smooth exhaust hookup it seems to small) I have tried tapping it with a rubber mallet but I dont want to crush the pipe. Any suggestions?