Heater Core

I cant beleive it 66 put it alll back together and it wont switch over to defrost ,i feel maybe i have put the heater box back together in correctly .I have A/C .The flap at the bottom opens and closes were the defrost tubes go properly opens and closes but no air
.im puzzled

I'm getting ready to head out on a drive. I'll look at mine when I get back and see if I can help since mine is an a/c box also.