A couple of reasons for this 1) The heater on/off valve (if fitted) may not be opening fully 2) The heater matrix is blocked, impeding circulation 3) The cooling system may need to be bled to get rid of any trapped air (feel the pipes leading to the heater - they should both be hot 4) The main thermostat in the car could be stuck open - but with this the whole vehicle would be running too cool.
John
December 2007