2004 Chevy Silverado dual-automatic climate control system. . .?

[LONGHORNS1999]

I own a 2004 Chevy Silverado with a dual-automatic climate control system. Sunday afternoon and last night I noticed that when I initially turn on my truck, my a/c system works as advertised, then for unknown reasons the cold air stops blowing out the dash vents and starts blowing through the defrost vents. Any ideas on what the problem might be?

 

[Chevy4]

I bought 04 chevy couple months ago, my defrost blows cold even when I turn temp to 90. I thought it was the blend door actuator, replaced it and it didn't fix my problem. Was wandering if you had found anything?

 

[zora]

Only the defrost blows cold when set to 90? If you switch to dash or heater it still blows cold?