Why am I only getting cold air when I turn on the heat?

I am having a problem with my heat. Checked everything out, no leaks, no overheating, got a new thermostat and bleed the valve for air bubbles. Only one o my vents gets semi hot and that the one on the drivers side. The 2 in the middle go room warm to cold every time I have something done to get it fixed and the one on the passenger side is freezing. The last time I tried to get this fixed the guy went in the car on the floor behind the controls and opened the vent the 2 vent in the middle began to get warm again but eventually went back to cold after driving 10 min. The vent on the end was cold. Now I'm being told that I need to get my climate controls replaced because the vents are not opening up. Before I buy could this really be the problem.