First check if the flap which changes from hot to cold in the heater box is activating when you move the hot cold switch, if thats ok Try flushing out the heater radiator and back flushing it, the narrow water ways clog up with shite, this may not clear it though depending on the blockage, I've filled various ones in cars with brick cleaning acid for 5 mins to break the crud down then flushing again with good effect but new heater rads are cheap enough anyway and easy to replace