Installed new PEX. I have new PP 4 Sided valve. Hot left... Cold Rt. Shower up. Plug down. No hot water comes out of valve body. Cold water comes out of valve body. All "guts" removed and no hot water or left side. Hot water gets to left inlet, but does not come out valve. Could there be some kind of casting defect?

Close and Open water valve (angle stop) under the toilet. See if it helps. Back to you. Milo

Make sure there is no debris in the hot port of the cartridge. Is there a shut off at the mixer? Open?