Sometimes there will be two countersunk screws holding the drum onto the axle shaft flange. These are placed there to keep the brake drum from falling off when you want to remove only the tire. Assuming you have those out, more heat and more beat is about the only solution. If you have access to a large puller, you can also put a small amount of tension on it with that, but you can't go too crazy with it because you'll crack the drum if you pull too hard.
I don't know what you're using for heat, but a torch with a rosebud tip might be what it takes to get the job done.