1993 Lesabre heating problems

Hi, when I put my heat on in my lesabre (I have ACC), it only comes out on the windsheild, and the feet, none at the vents on the dash... Anyone know why this is?

AC works as it should.

Check your vacuum line that goes into the black plastic container thing on the firewall towards the driver side (in the engine compartment). It might be unhooked or leaking.