I finally figured out what was going wrong with the potato bags.

I had two women who scortched their potato bag - asked them some questions and found out both of them had old microwaves - Nancy's microwave is almost 25 years old and Jane's is almost 20 years old. I contacted our Sears appliance manager and was told the old machines have hot spots in them and that is the reason they scortch. - that and the fact that they don't have the turntables so I have printed out a disclaimer letting my people know - that if they are giving it out as a gift, make sure the microwave has a turntable! It doesn't have anything to do with the thread or the fabric not being 100% cotton either... I use the prequilted fabric top make my dog's rice bags and use them at least 3 times a day with not effect at all. I din't think about it, but it makes sense