There are many reasons why this could be happening, physiological or otherwise, which is why I recommend you speak to your doctor about this since he'd be able to pinpoint exactly what the issue is and if there's a deeper problem that needs addressing.

What the comment below is referencing is death-grip syndrome, which is not medically recognised as a thing, but I don't dispute it personally.

For me personally, it's how I discovered my asexuality. I felt the sensations, I just wasn't perceiving it as pleasurable at all. I'm relaying this in case that's what you're feeling as opposed to total absence of feeling. And I would encourage you to test that by asking yourself if you feel any sensation down there when you masturbate.