As strange as this sounds, try tightening the other screws little by little and then try loosening the stuck one. Loosen all screws in a star pattern similar to a wheel bolt pattern One I bought had two stuck screws and that along with some WD-40 did the trick for me. The rubber half ball puts strain on the screws and causes them to bind if undone one at a time.
Sent from the crapper using Fapatalk