Check coolant level in the expansion tank when the engine is cold, don't wait for the dash warning.
Top slider: Windshield vent flap control
Middle slider: Chest level vents flap control
Bottom slider: Foot well vents flap control
Turn driver & passenger dials to warm & turn the dial for the vents above the radio to heat as well......?
You should get tons of heat from the windshield & floor vents when the sliders are open.
The rear vents that are mid/knee level do not blow hot air, they are fresh air vents. The rear passenger pointing vents under both front seats should blow hot air.
If you get low air volume even with blower on full blast, your 'micro filter' for the hvac system could be clogged up with dirt.
*Check your coolant level*