[Mark_T]

I hate to suggest this and I truly hope I don't offend you, but you are turning the screw counter-clockwise from the perspective of the screw head, right? Cuz that would be clockwise from the perspective of the guy sitting in the seat. Like I said, I don't mean to insult your abilities, but that would be an easy mistake to make.

The fix, once you get the wheel off, is upgrading to a three-spoke wheel. Those four-spokers are notorious for sagging on to the horn switch.