Part of the problem is that the bottom of the mug is thicker and takes a bit longer to get to the 400 f. Also the very top of the mug can be your press not getting hot enough at that outer edge or pressure as well. I keep all my designs at least a quarter inch from top and bottom and don't have those problems. I tried the all over approach many years ago when I started and decided it wasn't worth all the bad prints and adjusted fire and haven't looked back. There may be other reasons for your problems and I hope others will chime in but those were mine.I got mine from a different source. It could still be bad coating on yours too. Mine only happens on the bottom of the mug , even when i put it in backwards. This is with mugs from Coastal Business, Image Right paper,Cobra Inks,Mpress Mug Press @ 400° F and 180 seconds. Here is what mine looked like.