I wonder if you got a duff batch of pipe Gary, I've just measured the last lot I used........
I've found with the Flaremaster, you must file the end of the pipe completely flat, you cannot simply use a pipe cutter to part off the pipe, and ensure both halves of the die are completely aligned, and the gap on the jaw when the backnut tightened at 1mm.
I've never experienced any issues with pipe slipping through the die, and always managed to produce good flares on kunifer and steel.
Brian