Honesty, give it a good it with a rubber mallet where the rim and tire meet. Go side to side topto bottom. That should do the trick. If that don't work try grabbing from behind the rim and pulling from side to side.
My old VW GTI had that problem. My mechanic told me to put a little grease on the back of the centre hub of the rim. This helps the rim from "sticking".
I have been doing this for the past 15 years and have never had a problem getting a rim off.