I am having trouble popping the bagged microwave popcorn in my Sylvania Convection Microwave Oven. I have tried different brands and different times but still the popcorn doesn't fully pop. Only half the bag or less pops and if I try to make it pop long enough to make it fully pop it burns. If anyone can please let me know what I can do I'd greatly appreciate your help. It has a popcorn button but it doesn't work even if I use this feature.