Two things that come to mind. 1, Make sure that rad pressure cap is good and not leaking, if it is you won't get the proper coolant temp to open the thermostat at the right time. and 2, you may have a blocked or plugged heater core. Make sure that the temp of the two hoses from the heater core are both hot to the touch, this means that the heater core is not clogged up. I have always run the engine, turned the heater on full when topping off the coolant to make sure there is no air blockage in the system after changing thermostats or hoses. This is not gospel only what I have found on my trucks when I had a heat problem. Good luck.
