The heater core may be clogged (corrosion and/or phosphate scale) and need flushing.

You could also have an air lock. Did you change the antifreeze recently or have or have had low antifreeze level. Try topping it up then letting the car run for a while with the antifreeze pressure cap off. You may see the level go up and down. Thermostat might also have to be replaced. If those things won't help your heat problem you may have to drain all the antifreeze out and put it back in pouring it in slowly.