I made a deer meat roast and it turned out tough,...?

I put it in the crock pot and put seasonings, potatoes and bacon in there and cooked it on high for 4 hours. It was as hard as a rock, I have made it in an oven and has turned out well.. Most meats I cook in a crock pot turn out so tender that you can flake it with a fork. Anyone have an idea what went wrong? I am just learning to cook with deer meat. I have made some things with it already and they have always turned out well.