While the vast majority of people with nipple piercings will experience enhanced sensations, it is possible for minor nerve damage to cause numbness.
One reader recounts their experiences, writing,
"I had my nipples pierced horizontally, at a 10ga, and due to the my work situation, they were constantly getting banged around and eventually ended up rejecting, leaving a lot of scar tissue. I performed oil rubs, and sea soaks in an attempt to break down scar tissue. I thought it had worked.
About 6 months later, I had them re-pierced, at a 10ga again, and aside from the initial sensitivity, I thought everything was fine. Fast forward a month or two, and I find my self in a situation where my nipples were being played with, and it suddenly struck me that I couldn't feel a thing; my nipples had gone numb from the point where the jewelry enters/exits, up to the tip.
It's nothing major (I don't like when people touch my nipples anyway), but it goes to show it is possible."