Hi - I have an '07 Altima and recently my air vents have not been blowing air out when on the highest setting. When I start my car everything works fine. Then after being in the car for a while the vents gradually stop blowing air. It sounds as if the vents should be blowing hard - but no air comes out. The air is cold, vents are cold to the touch....just no air. Any suggestions what might be the issue? Thanks.