The shoes are rusted to the drum.
Kroil and some soak time should get it. It will ruin the shoes, but they need a retread now anyway.
Once it's soaked for a few days tighten the star wheel (adjuster) first then back it off. Hopefully it will still move or you may have to grow the drum slightly with heat to get it off. Just remember cast iron doesn't like heavy banging, a few heavy but careful taps can help if it's almost there.