Probably the blend door to the heater core is stuck. It can be hard to get to, most are behind the dash. There are at least (2) two blend doors. One is for the heater core the other is for the vents.
We had a Ford Windstar van we had to move the door by hand every winter. Ford says you have to remove the dash to change it, the cost at Ford was very high. It is a very common problem with all Fords.
Hope this helps.