Why Aren't My Nipples Sensitive During Sex?

Hi Betty,
After being with my husband for a few years, and a few other men prior, I have noticed that my breasts are not sensitive during sex. My nipples are the same way. The only time I have feeling, and usually tenderness, is "that" time. I've done some research online and have found nothing. I might not have dug deep enough, but I'm curious as to why my nipples are no fun for me.