Some people just aren't sensitive in that area. I'm not. I usually can't even feel it either and I don't know unless I see it!Lol. I suppose it sucks because we get nothing out of them being played with, but since I never really had feeling there it really doesn't bother me.
I doubt it would affect breastfeeding. Just because they aren't sensitive doesn't mean they don't work properly.

I am a 16yr old girl. I also have this problem!! I find it very embarassing :/ It could be 10degrees outside and I could be naked (just an exaggeration lol) and not get hard, or feel a thing! It produces an awkward situation, a guy almost expects a hard nipple, and unless I speak up and say somthing, he assumes I am not turned on. But i always am!
I have done a bit of research. It could be a Sexual Arousal Disorder. If you were abused in your childhood or suffered trauma when you were younger that could effect this. I am not abused, I believe mine comes from seasonal depression and hypothyroidism (an underactive thyriod) which both have been linked to little or no nipple sensitivity, also if you do not become 'wet' easily you could also have a SAD.
I hope this helps you!!