1999 Ford Expedition Heater

When you turn it on, does it blow air? Just cold? No air?

If you feel air:

Feel the heater hoses after it has come to temperaure.
Are they both hot or one colder?

Okay, either there is no coolant (air pocket, flow problem) going thru the core, or it is not coming to temperature (thermostat).

IS the coolant level remaining the same? You are not having to add any?
IF the coolant was crudy to start with, there may be a restriction. I have seen cores plug right back up, but not immediately like in this case.

Sometimes you can try jacking the front of the vehicle WAY up to help bleed our an air pocket.
Fords can be tougher to bleed out. There is a tool that works well called an Air-Lift vacuum unit to remove air also.

IF the thermostat is stuck open it can take an exceptionally long time to warm up.
https://www.2carpros.com/dia/test_thermostat.htm

On very rare occasions I have seen water pump fins eroded and no longer able to push the coolant.