YotaTech Forums - heat control issue

I'm Fabio From Italy and I've got a 1999 UZJ 100.
Since I started attending the forum, I have always read your comments and I really appreciate your technical answers regarding the maintenance of the LC. So I would be very happy if you could help me in solving a problem that I have in my car for some time. Since I had bought my LC about 4 years ago, the front heat ventilation never worked correctly while the rear one operated well. So, I and my mechanic replaced the engine radiator, the thermostat and T pipes. We washed the entire cooling system and used a water solution with a rust remover as "CLR" to wash and flush the front cabin heater core.
After that, front hot air flow restarted for only a few months. In fact, now the problem has returned as before. The heat ventilation does not come out from the front vents even if the heat knob is set to the max. Even, when the temperature knob reaches maximum value in manual setting, the air flow decreased in comparison when it is set to a lower temperature. All tests were carried out excluding AUTO but using only MANUAL mode.
Now I'm going to reflush front heater core using a product like CLR but in this occasion I'd want to leave it inside the core for a few hours before flushing it with fresh water.
What do you think about a problem on blend doors actuator or in a clogged heater core?
In your opionion, is there anything else to do?
Thanks a lot for your attention and helping me!
Bye
Fabio