It sometimes helps to kind of squish all the popcorn together in the bag into one mass.This helps prevent hot spots from forming.You have probably noticed how your food is hotter around the edges,and cool in the center.If you get a small quantity of popcorn off to itself,away from the larger mass of popcorn it will get pretty hot.Another problem I've had is the bag not staying sealed up long enough to inflate the bag(sometimes caused by the bag not unfolding properly as the popcorn pops,sometimes caused by a bad seal on the bag that opens to easily), this seems to burn the popcorn to.I think because the popcorn isn't able to move around,and this creates hot spots that burn the popcorn.You can help this buy sort of opening the folds in your bag so it inflates a little easier(not breaking the seal that would cause the back not to inflate).Your microwave is fine.