The pressure balance spool may be stuck. This might be internal to the cartridge, so changing it would have fixed it, but in some models, it is separate (don't know on this one). The valve expects the pressure on both sides (hot and cold) to be the same, and if it isn't, it will start to shut down the supply (and can turn it off).
What kind of water heater do you have? A tankless could have significantly lower water pressure than a tank type. Do you have a hot water pressure issue anywhere else? Are the inlet supply water pipes the same size for both the hot and cold? Is there any galvanized steel piping in there (it can rust out from the inside)? If the cartridge is removed, and you turn the water back on, to you get a good stream out the the valve body on both sides? Some valves use filter screens on their input. It could be plugged up on the hot side.