Couple of things.
Does it have a grease fitting and is it greased?
How about the way you have it set on the tractor. Is is fully engaging the ground or just skidding along? Sometimes when it's not completely down it does that. The only other thing I can think of is when the cutter's weight is sitting on it, there maybe some interference with the wheel shaft and the bearing or pivot tube. When you have it off the ground, try pushing up on the wheel like when the cutter's weight is pushing it up, then rotate. See if there's something goofy like a burr or something.