Originally posted by Nieklucz:

I don't know, I fell out of 4 pieces of skin in a row and I stopped playing because I thought it was a bug.

Probably just bad luck, then. The Wiki says, scrap iron has a 16% drop rate, leather scraps 4%. So far, getting 4 leather in a row isn't that improbably. Anyway, good luck on your iron mining, man. Don't forget to check the chests in the crypts, they often have tons of it.