There are several possible causes of "Warm but not hot" depending on the vehicle.
1: Thermostat (Engine t-stat) failed open, thus engine not reaching normal operating temp, thus heater not reaching normal operating temp. History: My girlfriends car on the coldest day in living memory had this problem.. I figured when I got to church it would be warmer inside... Wrong, Fr. is giving his state of the parish sermon "So thanks to Greyhound the boilers are paid for,,,, They don't work but they are paid for". FORTY BELOW your choice of F or C)
Second is a clogged or partially clogged heater core.
Next is a 2 parter, A: A faulty heater control valve not fully opening, (NOTE this may be caused by low air pressure or vacuum depending on the system) or "B" faulty air dam (A valve for the flow of air) same note.
Finally the air conditioner may be stuck on. (Clutch bearing siezed (happened to me) or short circuit)
Home is where I park it!