This complaint 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 properly 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 has chronically poor drying, if possible, have the vent moved to where it exits a house wall near the dryer.
The vent should be made of 4-inch aluminum rigid duct and elbows or, where absolutely necessary, flexible metal (but not flexible thin foil). Do not use flexible plastic duct—it restricts airflow and is combustible.
The dryer should vent outside—never into a crawl space, wall, ceiling chimney, or other type of flue. The bottom of the exhaust hood should be located at least 12 inches above the ground.
Here is what to do:
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. Note: 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 later if the dryer has one (read your owners 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 your problem, you may have a problem with your dryer’s heating element or thermostat. Call an appliance repairperson.
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