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But the fact is that the only real variation in nipple sensitivity between genders is negligible. The difference lies in mentality--women are more psychologically prepared for the experience, whereas most men tend to come into the game years behind women. In most cases, due to the effects of estrogen, females have more pronounced nipples and larger areaolae than men do. But the size and shape of the nipple has little effect on its sensitivity. Though the larger the nipple, the greater the variety of ways it may be played with.
(NSFW - explicit images)
In response to a girl compolaining of uneven nipple sensitivity another site suggests:
What may help increase the sensitivity of your right nipple is simply stimulating it on a regular basis, perhaps while you masturbate. You may be able to awaken and develop the nerve pathways there through use and familiarity.
It sounds like the only thing that's gonna cause a permanet change in sensitivity is nerve stress/damage or a psychosomatic change.