I agree with the other respondents regarding why glazes run. Over firing is the usual culprit, but there may be one other villian at work. I have had glazes run when the bisque firing was over-done. Because of the way the clay vitrifies during firing, too much heat at the bisque stage will actually "glaze" the clay without physically
applying any glaze. Applying glaze over the pot at that point is like trying to paint glaze on an already finished piece--it slips off! Check the firing temp of your clay and stick closely to that temp during both bisque and glaze firing, and you shouldn't go wrong. Hope this helps.