Ensure you have the full 240vac coming to the dryer. Also, check the vent system all the way out to ensure you do not have a clogged vent or damaged or crimped vent. Air flow is the most common issue with in efficent drying.
With the proper power to the unit, again ensure the vent is of the correct size and not crimped or clogged anywhere. Make sure the heat is coming on, pull the machine out from the wall, take the vent loose from the machine, and run a load. If it heats and dries properly, then the issue is the size, leghth or condition of the vent. If it does not, then again check the thermostats, the valve coils, ignition components.