Too much glaze, using a runner on an entire piece, or not wiping off a high enough foot can all cause this to occur
Too much clear glaze can cause your piece to look milky and can also cause surface cracks in the glaze
A perfect coat of clear glaze on a sgraffito piece
Sgraffito slab sculpture without clear glaze (fired twice) Notice how the color changes but the surface remains flat rather than the glossy coat clear glaze adds