Some more thoughts............
I doubt you have an "airlock in your heater core since it s at a lower point in the cooling system. Any air would work itself out in short order. I believe there is a couple bleed valves in the engine compartment cooling system- check where the upper radiator hose joijnd to the engine at the thermostat housing. The Lumina engine sets higher than the radiator, so it has a harder time burping out naturally.
As stated already, let the car warm up and go feel the heater core hoses at the firewall in the engine compartment. If they're both hot, then you have good heat going through the core. Heater core could be plugged/restriceted though. If they're not hot- check your thermostat for being defective or installed wrong.
Air has to pass through the AC evaporator and the Heater core before coming out your vents. The AC evaporator is first in the incoming air-flow (in the dash ventilation ducts) and can become clogged with a mat of dirt and dust over time because of the moisture it produces. This will eventually restrict enough airflow to not adequately heat or cool the inside of your car. Usually you'll notice the vents have a weak airflow even when the fan is on high. Some folks say to replace it, some sy buy the spray faom cleaner for it, I've had luck getting behind it and brushing off the back with a toothbrush (throw awy toothbrush when done!). Cleaning it saves opeining the AC cooling system and works pretty good, but doesn't work quite as good as a replacement obviously.
Good Luck!!!