If cleaning the head didn't help and you have plenty of hot and cold out of the tub faucet, then the only thing left is the diverter valve.
It is probably plugged up. Depending upon the type of faucet, one has the pull knob on the tub outlet that stops the water and forces it up the shower. With that type, you would have water leaking by the tub outlet. The other type is part of the faucet assembly in the wall. Explain what type of faucet and how many handles, but you may need to pull the single faucet handle and remove, clean, or replace the cartridge or what ever part your brand of faucet has. Try putting a rag over the tub outlet to try to force the water.