Check your heater hoses... They should both feel warm to the touch if your engine is warmed up and the heater turned on. If not, then you are not getting hot water to circulate through the heater core, possibly due to an air bubble.
Try to bleed the air out of the hoses and the heater core, and see if your heating situation improves. Don't forget to refill your cooling sytem after bleeding the heater, or the problem could come back.
