My guess is that you have been seeking out groomers as opposed to going to a vet. Considering the undeserved rap on the breed, many groomers do not take into consideration the individual personality of the dog. If you have a vet and they will not clip your pets nails perhaps it's time to make a change.
Because people all over the planet are idiots... and there is a huge discrimination against pit bulls because of what people like Michael Vick have done to them. Have you considered doing it yourself? It would be a lot less stressful on your dog (although a LOT more stressful on you!) Good luck.