If there is no difference between the new and old cartridges then it seems like there is some kind of obstruction in the valve body. Since the problem is with hot water the obstruction is on the left side. I'd get a bright lite and look into the openings. I have probed with a very small Phillips screwdriver to find something. Another option would be a straightened paper clip with a 1/8" 90° bend at the end to make a hook. My last ditch would be to leave the valve disassembled and supply hot water at normal pressure. Might be a bit messy but better than replacing the whole thing. I really hope you have good luck and solve your problem.