There are a number of things to check for poor heater performance on your 2004 Chevrolet Tahoe. Low coolant level, stuck open thermostat, restricted heater core, faulty operation of temp control actuator, etc. There could also be fault code(s) stored in the A/C control module. Any stored code(s) would be related to the electronic operation of the temp control system. For more information please see the RepairPal article on heating systems - http://repairpal.com/i-have-no-heat-what-could-be-wrong.
You could have a bad damper actuator. You could have more than one bad. The actuators have sensors in them that tell the computer what position the damper is in. When the system gets wiggy, the computer tries to send the damper full open and full closed to recalibrate itself. If it doesn't move fast enough or far enough the computer assumes the actuator is bad. Often times the plastic gear inside the actuator gets a hairline crack. Its enough to allow the metal shaft to turn in the plastic gear. This messes up the calibration. Its also like having a stripped gear in a transmission. The motors turning but the parts not moving. Sometimes it keeps trying enough times and it gets the damper to move some and you eventually get warmish air, but not hot. You could have a bad blend air door actuator or a bad recirc damper actuator, or both. It's not uncommon to find multiple actuators bad.