double check and make sure you didn't miss a bolt. If you got them all, you just have to wack it harder. If it has a bead of silicone on it, they can be pretty well stuck. You could also use a pry tool and pry one edge up ( do not mar the gasket surface with the pry tool ).
I had to do valve covers on a friends truck and we had to beat the snot out of them to get "em off.
Use a thin metal paint scraper and, with a hammer, work your way around the silicone joint. It will take some patients. Ford uses some pretty good sealer on those and theres also a 50/50 chance you will need to put a new one on it. Good thing is the oil pan is pretty cheap.