Most modern cars engage A/C automatically when the windscreen defrost mode is on. With the heat set on high and the A/C engaged, the system acts as a dehumidifier. Set the heat at its highest position and check that the A/C is engaging when the defrost mode is active. Depending on the car, you may see that the A/C is active by an LED on the control. You may also year the compressor engage when you switch on the defrost mode. You may have a problem with your A/C system if its not engaging.
Another possibility is too much moisture in the car to begin with. For example, if the floor mats are very wet from tracking in mud and snow the last time you used it, it can take longer for the moisture to be removed from the air. Ensuring that you are selecting outside air and not recirculating air will help.
If the car is older, other problems can be poorly fitting doors that leak water or drains at the base of the windscreen clogged with leaves and dirt leading to leaks. If you smell anti-freeze you could have a leaking heater core which will emit steam into the air you're trying to dry.