why does the heater only blow on defrost, it used to work but won't shift to floor heat or vent

you must have a vaccume hose off or your blend door not opening right or not at all? check all your vaccume hose runing under dash around your vents and under dash. it could be the switch itself. i'm betting on hose.

you may have a vaccume line off or your blend door is sticking. when you try to put heat to the floor do you hear a noise like it's opening to switch to floor?

Model year?