Hi there tried the search but cant find an answer to this problem

I have a A4 1.9 tdi nice motor few issues but working on them but one i cant figure out is when i turn the blowers on i get air from top vents and bottom vents but when i chose the main vents i get nothing no air but the blowers are still blowing.

is this a comon fault and if so any help on fixing would be great and most apreciated.

 

Possibly a failure of the blend doors inside the dash or a faulty climate control head unit.