[jhv48]

If you try everything else, you might just have to resort to this:

Drill a small hole in the center of the cap, just large enough to insert one of those needles used to fill up a basketball from an air pump. Obviously, you might end up replacing the center cap if the hole bugs you.

Now, insert the needle into the hole and turn on an air pump to force air into the cap. If it isn't glued firmly in place, it should pop out.