While it might seem opposite of cooking most meat dishes, UNDERCOOKING is the biggest culprit of tough pot roast! Pot roast is made from a large, tougher cut of meat. If the meat is undercooked, it doesn't allow the connective tissues to break down and tenderize the meat. If your pot roast seems tough or doesn't shred easily or fall apart, you need to stick it back in the oven (or slow cooker) and just patient. Pour yourself another glass of that leftover wine :)