You might have air in the system. Try bleeding it by the following; Let the car cool down then remove radiator cap
Turn the heat all the way to hottest setting
Start the car and allow to reach operating temperature
Fill your radiator and reserviuor.
Take for a test ride and see if it solves the problem
If it does not you may have problems with the heater control actuater valve which controls the flow of coolant into the heater core.