the normal reason on a car with some miles on it is that there is a ridge on the inside lip of the drum where the shoes don't touch. Some drums have a knockout plug that is about an inch long and a quarter of an inch wide it will be rounded on the ends. you have to use a punch to knock it out then use an adjustment tool to back the shoes off with the adjuster. The little wheel inside with the teeth on it. you will also have to hook the adjuster lever with a small hook and pull it away from the adjuster wheel so the wheel will turn.
a piece of 1/8 in welding rod with a little 90 degree hook on one end and a loop to hang onto on the other works good. Once you get the shoes backed off the drum should come off.
If it is rusted to the hub. I would suggest spraying on some PB Blaster and the using a big hammer to persuade it a bit.