I've never been able to master the harmonica, even though it's supposed to be easy to learn.
I can't play it either and I don't usually struggle to pick up instruments. My lips would go numb when I tried to tongue block and I couldn't position myself correctly. I can jam on the electric and acoustic guitars. I can play the piano and organ. My drum kit is the one I play most often. No harmonica though, definitely no harmonica.