Heater question

[James R]

Hello all. The left hand lever of the four levers controlling the heater (i.e. the one controlling how much heat you get for air going upwards) only goes down about a 1/3 of the way down and then stops. Its being blocked by something and I don't want to force it.

What do you reckon it is and how should I fix it ?

Thanks as always for your help.

James