I bought some muffin tins at the dollar store that did the same thing. (it's especially bad if they are tinned)
I use 2.5 or 3 ounce stainless steel "condiment cups" for ingot molds. The ingots weight about 1.5 pounds each -- more when I cast soft lead. And the ingots knock right out.
"The only way to stop a bad guy with a gun is with a good guy with a gun"