We have a leaking shower. We have replaced the seat and stem on the water but it still leaks. Also the diverter does not shut off all the water.
To oversimplify things, a shower control has two parts: the fixed part attached to the pipes in the wall, and everything else (which is usually the cartridge and possibly some O-rings if they are separate).
If you've replaced "everything else", then the problem is the fixed part in the wall.