Thank you very much for your answer.
I follow your advice and I did the cold pulls (atomic method). It looks like there is not much of a burned PLA, dirt or dust or whatever there (see photo).
So, I checked the diameter of the teflon coupler. After unscrewing the nozzle I tried to push the 3 mm drill from below with the blunt end, which, as I measured, has 2.9 mm diameter.
And it not fit - it wont go. I cannot say how much the coupler hole was smaller, but it was not much, maybe about 0.2 mm. So I drilled it with 3 mm drill.
After all of this cleaning and drilling I must say it got liitle better. It's not like new, but I can see positive difference.
I read somewhere here, that even 4 mm drill can be used. I wonder if I should use 4 mm drill.
The other printer (the older one) is still in service. I was told that the clogged nozzle was cleaned and the coupler was changed to new one. In few days I will get the printer back and check if it solved my problem.
Anyway, again, thanks for your help.