There is either a problem with the duct system where it may have come oose and is leaking. More likely is that an air diverter door is not being actuated.
I am giving you a walk through diagnostic for your situation. Just use the link below to get to the page;

http://www.2carpros.com/articles/low-or-no-air-flow-from-vents

Answered by DrCranknWrench (expert)
3,292 answers provided