The complaint that a dryer takes too long to dry is most often caused by the dryer’s vent being blocked with lint. Dryers need to be properly vented or the moisture in the air can’t be carried away. 
Vents that go out the roof are particularly problematic—the dryer may take far too long to dry clothes, and the vent pipe may even drip water that can’t be exhausted. The minimum size for a dryer’s roof vent is 4 inches. If your dryer is chronically poor at drying, if possible have the vent moved to where it exits a wall near the dryer.
For most drying problems, perform the following:
1) Check and clean the lint trap.
2) Be sure the dryer isn’t pushed so close to the wall that it pinches off the airflow through the vent’s air duct hose.
3) Check the point where the air duct vents away from the house. Be sure plants, birds, or other objects have not blocked it. If the duct vents out the roof, it could easily be blocked with lint.
4) Disconnect the air duct hose from the back of the gas dryer and clean out built-up lint. For safety, first turn off the gas to the dryer, and then unplug the dryer; this means you will have to relight the pilot light (if the dryer has one) later; read your owner’s manual for proper relighting techniques.
5) If necessary, clean out the ductwork from the dryer to the exterior wall where it vents; this may involve disconnecting sections. One trick that sometimes makes this job easier (but only if the dryer duct is fastened firmly at all connection points) is to blow lint and debris through the duct and out the exterior wall vent using an electric leaf blower.
6) If the dryer still is taking too long to dry clothes, remove the lint filter and use a flashlight to check for internal clogs. Use your vacuum to suction out any blockages, if possible.
7) If the above steps don’t solve the problem, you may have a problem with your dryer’s heating element or thermostat. Call an appliance repairperson.
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