Had one oddball muffin pan do the same thing to me. turned it upside down and went to work on the bottoms of those cups with my framing hammer. Boy were they stubborn to let go. Was afraid the pan may have had tin in it and is why they stuck so badly. Didnt want it in my smelter. All of my really old muffin pans from thrift shops work fine. Bought some old ugly ones for about a quarter apiece and hit em with easy off oven cleaner after I heated them in an oven to around 250 degrees . Cleaned up real nice and polished them up a tad with an sos pad. Drops those lead biscuits like a charm. Wish my boolit molds would learn how to do that.