As Repair 2000 points out, when a dryer fails to dry clothes it normally has a problem with the lint filter or vent. Other causes for dryers that won't dry include issues with the heating element, temperature sensor or thermometer. Drying problems can also be caused by loading too much clothing into the dryer or by incorrectly setting the controls.
When a lint filter is full, air does not flow freely through the dryer, and this causes the dryer to work harder than usual. The lint filter should be cleared out after every use, and cleaned with warm, soapy water, especially if fabric softener is being used.
Keeping the dryer vent clear is also crucial for the dryer to heat properly. Clogged vents can result in long drying times, clothes that are still damp when the cycle is over and failure of automatic sensors to shut off. Even more important, clogged vents can cause premature failure of the dryer's motor or heating element and pose a real risk for house fires. The dryer vent can be cleaned using a long bristle brush. The outside dryer vent should also be inspected regularly, and all the lint inside cleared out. Obstructions in the ducts can keep the dryer from heating properly, as can kinked or twisted dryer vents.
If the vent and lint trap are cleaned out and the dryer's controls are set properly, the problem may be more serious and necessitate a visit from a repairman.