I had that happen a couple of times early on. Never really did figure it out except when I got a better heating system the problems stopped. It may be a case of it being too hot. I don't heat with a micro so I can't help you there. I add all my color, glitter, salt and scent after the plastic is up to temp. The plastic is still on the hot plate when I do that. Some guys add it before heating and swear by that method. Hope that helps.