I’ve pin point that the problem is with the actuator not turning enough to let the heat come through. I put a new actuator on it and it did the same as the old actuator. But I can turn the knob with my hand and the heat blasts through. What could be the issue? The truck is a 1999 Tahoe and I don’t think the actuator can be calibrated.
The options are a faulty actuator, a faulty control panel or wiring issues. Many times the supplied replacement actuator is not the correct part or it was installed in the wrong location. Since you stated that you can manually turn the knob (I am assuming the door shaft) and get heat I would start by making sure the replacement part is correct for your vehicle. If you could supply the number and brand I could double check it.
If you have a voltmeter and backprobe pins I could help you with some test procedures.