Some toothpastes can either cause sensitivity or help it get better. What kind do you use?

I've used a variety. (Right now, I'm using Tom's of Maine because of their no-cruelty policy and the fact that they don't use any sweetener.) None of them seem to make it better, though I haven't tried any of those that are meant specifically for sensitive teeth. I think that there's something more acute wrong, though, as it's really only one tooth, not my teeth in general.