2000 Yukon Heat not hot

Vehicle: 2000 Yukon SLT 4.8L Std heat controls, no diff betw passenger and driver. 3 knobs - L to R Fan, temp (just blue to Red, not in degrees) and far rt is moving the airflow around from feet to defrost.

Problem: Got a little cold here in FL last week and turned heart on. No go. The overhead is actually a little bit warm but nothing to write home about. Over the past year, I was hearing an occasional thumping in passenger dash but figured I'd deal with it when something broke. Still don't know if that was the problem.

So, I decided it might be the heater Blend Door Actuator. Removed the panel under glove compartment and also the center console. Took actuator off. Fussed with it a bit and plugged it back in and started the engine. I then moved the middle temp knob from cold to hot and the actuator moved from left position to rt position but I was unable to make it move again after this. unplugged it, plugged it back in, removed battery cables and HVAC fuse for 5 minutes, etc. With it off, I reached up and was able to easily rotate the spindle that opens/closes the door. I finally gave up and bought a new actuator from Auto Zone. Installed it and turned engine back on. The temp control now activates the actuator and opens/closes the door. Done deal right? Wrong! Still no heat!!

Temp gauge sits at about 200 F on the truck. 1 year ago had new radiator installed. 2 years ago had new t-stat installed when the new water pump went in.

Thoughts anyone? I'm thinking it might be the air bubbles I'm reading about in the heater core but unsure. Could it be anything else? I don't have any significant loss in fan pressure anywhere if that makes a difference.

Thanks!!

On my import truck, I got much more heat when I replaced the non-standard 180F thermostat with a standard 195F one.

Yes: bleeding out all the air from the cooling system is also important.

So I took it to my guy at Tires Plus and he carefully flushed out the heater core. Done deal. Immediate heat and heater core didn't start leaking. I wouldn't be surprised that some of the stop-leak I put in years ago for the radiator ended up in the heater core and was just blocking things up.

Thanks to Cusser for the response too. Appreciated!