On my Plus8 the wheel sat on a metal "frame" and the centre bolt had a centering disc that fitted into the wheel centre, through to a threaded hole in that frame. Thus centering and holding the wheel in place. No other bolts. There was a steel plate on that frame and the wheel sat on that so where it contacted the plate is where it might be sticking. If the wheel hasn't been removed for a long time then it could end up sticking. One good idea is to rotate the wheels periodically so you use the spare rather than leave it age and detriorate.
