The pictures show the problem just fine. What you have is a Delta ball-type shower valve, and it appears someone may have torqued the body, although the trim lines up. I am not sure if this can be fixed. It could be a defective ball, so the first thing to do is shut off the water, remove the handle and then disassemble the valve to replace the ball. Get a genuine Delta brand replacement. Possibly post a picture after you remove the handle and the chrome plate to see if we can spot something.