First, verify you thermostat is working correction. If the temp gage is rising to the correct temp and holding, you thermostat should be good.

Two, make sure your car does not have a heater core shutoff valve. On my car, there is an electric one that can get stuck closed. If it does not, which is likely

Three, your heater core could be plugged up. It may need replaced.

One off chance.... did you have any work down to the system prior to the heating problem? If yes, some models can get an air bubble in the system preventing coolant from circlating to the heater core. This air must be bled out. How depends on the model. Most do not have this problem, and it should have worked it's way out of the system if you drove it at highway speeds.