Navel piercings are unfortunately one of the piercings that don't always work well for people who are a bit heavier. It has more to do with the shape of the navel as you gain weight, the ledge where the piercing goes tends to round out a bit and doesn't support the piercing as well. Of course, if you have rubbing from extra weight, that doesn't help at all either.
Of course, I'm not saying this is the culprit, it could have been incorrectly placed, or you could have a nickle allergy (most places use stainless steel which is good for most people, but contains a small amount of nickle. If you are sensitive to it, then titanium is the best thing to use, as gold actually contains a bit of nickle as well).
I used to be a piercer, back before I decided I couldn't make a comfortable living off of it, and it was always a bit of an uncomfortable subject, trying to explain to some girls why I wouldn't pierce their navel. But as I said, it was always about the shape of the navel, just heavier girls are more likely to have a shape that doesn't support a piercing well. Some skinny girls are ill suited to the piercing as well though, so it really is just anatomy dependent.