89 cavalier heater problem

i have a 89 cavalier convertable z24 and im having a problem with the heater
i only get heat on the defrost and heater settings but when i move it to the vent all i get is blowing cold air. i have the bottem of the heater riped apart to see if every thing is moving right and from what i can see its working fine i don't what is blocking the heat from coming out of the vents it works fine on the other setting. i do not have a/c if anyone knows what might be wrong could you please tell me thanks