When we were trying to do the same for our girlfriend we ultimately had luck using tongue and vibrator at the same time…Either one on each nip or if you can pull it off both on the same one. Try to alternate and give the areas around some attention too if she likes that. Can be kinda like a clit where too much for too long can get kinda…overwhelming in a non orgasmic way. Girlfriend likes sucking and lite biting on them so that helped too. She likes lighter touch with the wand while for myself i occasionally like to press the wand up against the underboob and experiment with speed until the wavelength is just right to stimulate out to the nipple/areola. The indirectness of it can be a nice ‘surprise’ and keep things free ‘on top’ for whatever implement you wanna use.
Didn’t know that was common, i always feel weird telling people because it’s like a hard no for me and sometimes people think i’m joking or something. Sucks, i really wish i could enjoy seeing other people’s tums and cool piercings.