Probably some rust or corrosion between the hub and the rotor. You can try it a couple of ways. Heat usually does the trick, but you have to be careful with the torch and only heat the rotor and tap the periphery with a real hammer(ditch the rubber mallet). A less aggressive method is spraying the center of the rotor, hub and lug bolt holes with penetrating oil, tapping the periphery with that hammer, it should come free.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.