My 2008 Máxima isn't blowing hardly any air when its at feet and top vent mode. Also not any air in rear vents.. Fan in console sounds loud like it should be blowing a lot but not coming through vents. Works fine when vent mode is on head or feet only..

 

Here are your 3 possibilities...
Your diverter behind the dash is jammed in the defrost/foot setting.
The wheels below the dash vents are all set to the "closed" setting.
The wheels are set to open, but the rods that opens the flaps have popped out of their mounts and don't open the vents at all. Something I have had to fix several times on my car.