Some manufacturers of carbide insert dies (and other inserts, e.g., TiN, and TiC) recommend that the die not be seated tightly against the shellholder. The reason is that the insert is sweated (silver soldered or brazed) in the die body to hold it in place. If one adjusts the die so that it is very tight against the shellholder, it can either crack the insert (as already noted) or break it loose from its seat in the die. It is possible that certain designs might be more tolerant of tightening than others, so go by the recommendation of the manufacturer of your die.