if its white i think that's undersintered. oversintered would have come out greying.
things that can affect it: residue or moisture in the unit, this causes a sort of barrier for the heat. usually zirc breaks when theres some of this present. that said, zirc can sinter as low as 800 deg c so double check temperatures. if it never made it to the top temp, then it would still harden but the shade would be wrong.