Is it normal that I hump rather than "jack off?"?

Okay, I am a 15 year old male adolescent teenager. For a while, I've realized I just can't seem to ejaculate when I "jack off" but when I hump my bed. I don't understand what's wrong with me, and I'm worried. Everyone else seems to be able to do this fine, but I can't. Is it wrong? Or just really rare?