I have low or no hot water pressure to the downstairs bathroom. I've replaced the cartridges, valves and hot water heater, still no pressure... help.

How is the cold water pressure? Is this happening in all valves in the bathroom or just one, of course you may have only one? If you have replaced the water heater,the cartridge and the valve itself, that kind of leaves only the hot water pipe which I doubt. Check to see if you have a stop valve on the hot water line to that bath. If so, there is probably something clogging that valve, could be mineral deposits or could be solder.