A dryer that leaves clothes damp is almost always telling you about airflow or heat, and it is rarely failing in a way you cannot test for. Work through the checks below in order. Each one rules out a cause before you move to the next.
Start with the load
Check whether the load was too large or the items were too heavy. Towels and thick fabrics hold a lot of water. Check that the cycle matches the fabric. Then dry a light load alone and watch the time it takes. If that light load dries normally, the problem lies with the load and its size, not with the machine.
Clean the lint screen
Pull the lint screen and clear it with your fingers and a soft brush. Check whether it is coated with a thin film from fabric softener or dryer sheets. Wash the screen in warm water with a little dish soap, rinse it, and let it dry completely before you put it back. A coated screen restricts the airflow the dryer needs.
Check the exhaust path
Run the dryer for a few minutes and go outside to feel the exhaust at the hood. Weak airflow there points to a restriction somewhere in the duct. Look for crushed sections, lint built up in the elbows, and a hood flap that sticks shut. Compare the duct length and bends against the manufacturer's installation limits. Long, bent, or narrow ducting slows the air and keeps the load wet.
Check the heat
If the exhaust is strong and warm but the clothes stay damp, the heat source may be weak. Watch whether the drum heats at the start of a cycle. Electric models rely on a heating element and a protective fuse. Gas models rely on an igniter and a burner. Do not open the cabinet to test these yourself. Ask a qualified service person to check them.
Check the power and the gas
For an electric dryer, confirm it is on a circuit that delivers full voltage and that the breaker has not tripped. For a gas dryer, confirm the gas valve is open and that the burner lights. If the drum turns but no heat comes on, the heat side needs service rather than more time on the cycle.
When to call
Call a service person when the lint screen is clean, the exhaust is flowing, the load size is right, and the dryer still does not heat. Have the model number and the install year ready. The clothes dryer page on this site gives typical service life and when to plan a replacement, which helps you decide whether a repair or a new unit makes more sense for the house.