How is the exploration going little Gwyn?
Have you already tried adding weight to your clamps?
If not, consider starting from something not really heavy and raise a bit at every session to find your sweet spot. Always remember to take a break every twenty minutes max and rotate clamps everytime you open them x
EDIT: also, when your nipples are weighted, try to do some movement with your upper body. Bend it, snap it a bit, experiment and let us know