what would cause the wheel to get stuck on even when the nuts are off?

Give it a good swift kick on the outside side edge of the tire, assuming it is secured properly off the ground, don't kick it off a jack. A sideways motion might break it loose, and spray the hub with WD 40 maybe 5 minutes before

Rust and corrosion is the cause. When you get he wheel off, wire brush the hub area and paint it with never-seize.