I put 6 C4 on the road on Kharg Island, and when a tank drove over them I blew the C4 and I only disabled the tank
It also took me 7 RPG's to destroy a tank when I was capturing the forest in Caspian Border
can't imagine what went wrong with the C4
but for the RPG thing, a guy was probably repairing it
or you thought you hit it some times, when you didn't actually hit it
try hitting them in the back though. it makes a really big difference