Before you call an appliance repair company, run through this list. Four of the six causes are things you can check in ten minutes, and the most likely one is a clogged vent.
1. Restricted vent (most likely)
Lint in the duct or at the exterior cap. Test: run the dryer and check the outdoor flapper. Weak airflow or a flap that barely moves confirms it. Fix: full vent cleaning.
2. Crushed or kinked transition hose
The flexible hose behind the dryer gets flattened when the dryer is pushed back. Test: pull the dryer out and look. Fix: replace with semi-rigid aluminum and leave room behind the dryer.
3. Dirty lint screen or softener film
Dryer sheets leave an invisible film that blocks airflow even when the screen looks clean. Test: run water over the screen; if it pools instead of flowing through, it's coated. Fix: scrub with dish soap and a brush.
4. Overloaded or over-wet loads
A washer that isn't spinning well leaves clothes too wet. Test: clothes should be damp, not dripping, out of the washer. Fix: smaller loads, higher spin speed.
5. Failed heating element or gas igniter
The dryer runs but air isn't hot. Test: run on high heat for five minutes and feel the exhaust air at the cap or inside the drum. Cool air means a heating problem. Fix: appliance repair.
6. Moisture sensor coated with residue
Sensor-dry cycles stop early or run forever when the metal sensor bars inside the drum are coated with softener. Fix: wipe the bars with rubbing alcohol.
The order to do things
Check the hose and the screen, then test the vent airflow. If airflow is weak, get the vent cleaned before paying for an appliance call. Most repair techs will tell you the same thing after charging a diagnostic fee.