Checking air flow and venting hose
While your dryer is still running, and before you disconnect its power supply, place your hand over the exterior vent while it is running. You should be able to feel a strong air flow. If the air flow is weakened at all, check the following areas:
A common culprit for an impeded dryer air flow is a blocked lint filter. Remove your dryer's lint filter. It will be located either on top of your dryer or just inside the door. Examine it closely. Remove all lint from the screen. This should be done as often as after every load. Letting it accumulate can lead to drying problems and it can also be a fire hazard.
If you clean your lint filter regularly, check the venting hose. Make sure it is securely connected to your dryer and the external vent. Also, if your venting hose is kinked, it could create lint build up and poor air flow. Do a check of your dryer's venting hose to make sure it is not bent in an awkward position.
Unplug the dryer and pull the unit away from the wall. You are going to need to get in behind it. Remove the end of the hose that connects to your dryer. Take a look for lint build up, or any other sort of blockage. Also do a check for any holes or tears in your venting line. Plus check the venting line inside the dryer, and use a vacuum cleaner to remove any excess lint. Repeat this process periodically.
Reattach your venting hose in the shortest, most direct way. Avoid letting the hose drag down low, because those low points are prime lint-accumulating areas.
Most dryer venting hoses are four inches in diameter. There are hoses made of flexible PVC/Vinyl, but those types of hoses are not very fire resistant and they are quite prone to sagging. You really should use a semi-rigid aluminum vent. These kinds of hoses are much more fire resistant and much less prone to sagging.