Coincidentally, I just ran into a sticking problem when I switched from pouring lead to bismuth/tin alloy in a custom brass mold. Unlike shrinking lead, bismuth expands when cooling, & the sprue would break off leaving the 1/4 oz. jig head so stuck only a hammered awl on the break scar could release the casting. Candle smoking didn't help. Finally spent many hours leveling out high spots with Dremel cutters(manually like GL Man)& finish polishing with rubbing compound & a wooden matchstick, and Voila, problem solved, today in fact. Well actually, it took about four attempts & a lot of 4 letter words...
So, I would conclude that sticking castings, especially lead, are the result of mold cavity defects which should be repaired. Think of freezing water in it's plastic bottle, no way the ice can be removed with the ridges and necked down opening. Do-its generally don't have serious design or production defects problems, but I've gotten a few with excessive "orange peel" ruff textured cavities that needed smoothing & polishing to release castings smoothly, mostly bismuth alloy, tho a really ruff one can grip lead pretty good too.