You could try a mix of mostly alcohol and a little penetrating oil. Just a wee bit around the edge, and let it sit overnight. If that doesn't work, I was able to get one that was really stuck on by placing a screwdriver into the slot at an angle and giving the screwdriver handle a few taps in a counter clockwise direction w/ a piece of wood or a small hammer.
It doesn't take much, the tapping should break it free from being stuck. As soon as you see a little movement, your tool should be able to spin it right off.