If we can assume the hot water line goes to the fixture, and that could be a question with todays quality control, the problem is within the fixture. Most fixtures of the single handle type have two port inside and a barrel that moves across these two port to mix the temperature.
First thing would be to open up the fixture and see if things look logical inside. Look for debris in one of the ports. Make sure the barrel can in fact rotate enough to open the hot water port. I say this because many single handle fixtures have temperature limit adjuster built in them. If you find a notched ring just under the handle that controls the hot water flow and thus temperature. Each notch is generaly 3 degrees increase or decrease.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles