Back Flushed heater new t stat coolant full get heat but not very warm
Look for a malfunctioning blend door.
With key on , press and hold the power button and recirculation button at the same time for over 5 seconds. This will calibrate the mode doors if it is an automatic climatete control. It will blink lights and last 2 or 3 minutes.
Is the dash coolant temp gauge showing a normal operating temperature? Do the two heater hoses feel hot?