Ford mustang convertible. The air conditioning only will not comeout of vents, only windshield defrost vents.?

No matter what setting I use (defrost, A/c, max a/c, vent) the cool air only comes out of the windshield defrost vents. I can feel the cool air when i out it on a/c comming out of the defrost vents but nothing out of the normal face or leg vents. Anyone know what the problem is?