Might be hung up on the hub. Try shooting some Kroil or PB on it in that area, and perhaps lightly sand the hub ti take any rust off. One trick that's worked for me is to get a couple of small (about 18" ) prybars to get in between the drum and backing plate to apply pressure. I'm not talking about any major force here, just enough to get some pressure to help pop it off. Then give it some solid whacks, preferrably using a brass hammer. That usually works for me with no damage to the drum or backing plate.
