Some people simply have less sensitivity in their chest than others, regardless of their mental feelings about their chest. I don't bind and I've never had much feeling, and neither does my female friend who enjoys her breasts. If I'm not looking and someone tries to play with my nips, I honestly usually don't even know they're doing it, and my friend is the same way.
If you used to have feeling and it's started to go away over the years, it's probably from the binding. Unless you really enjoyed having sensation in your chest, I wouldn't worry about it.