How to get a jammed towball out of the trailer hitch?

Hi,
I recently used my garden trailer and it refused to come off the car's tow hitch. I unscrewed the towball, so at least I can drive around without towing a trailer everywhere, but does anyone know how to get the towball out of the hitch on the trailer?
(brute force and swearing doesn't work, and its been well sprayed with WD40, more than 24 hours ago).