Climate control expert needed.

[Tim51]

So here is my problem. Air only blows out the vents when the dial is positioned in the blue area. When I hit the defrost button or move the dial to the warmer temperature settings, it is like the vents close. The fan is still running and trying to blow air, but there is only a trace of hot air coming out the vents. If I move the dial back to the blue section(not in the defrost mode), the vents open back up and it blows hot air for a few seconds then starts to get cold. How can I get hot air to blow out the vents? Thanks for the help.
Tim