Blend doors gets stuck sometimes, how do you fix it

So I think I am stating this correctly. The knob to the right of the fan is the knob that controls were the air goes (feet, chest, defrosters, etc.). I think is the blend door but sometimes I can't get the knob to turn. Any ideas on how to fix this?

 

Have you ever had your dash apart? Maybe there is a wire or something else interfering with the linkage.

 

I have not however I am not the original owner. Anythings possible I guess. Just curious if this is a common problem and if anyone knows how to fix it.

 

Very common problem. Mine is stiff when cold until it warms up a bit... haven't dug into one very far for this concern tho. I'd start with removing the heater control aND see if the binding is from th knob or if it's the actual doors in the box