Reading this reminded me that I have had this happen in the past and I
can not tell you what the variable seemed to be-I am currently doing an
underglaze assignment with my students, for the same reasons you mentioned
and now I realize I'd better keep my fingers crossed-I'll be interested to
see what suggestions anyone has....I'm doing low fire white clay as well-and
we're applying to bisqueware-several coats and clear glazing as well.
Good luck-I too, have felt this pain!