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We always troubleshoot this problem by going outside of the house to where the dryer vent is. Have someone turn on the dryer and watch and feel the air that is being blown outside.
If this air is not warm/hot and not coming out with a good breeze, then the problem is a clogged or kinked vent hose. Go back inside and disconnect the vent hose from the back of the dryer. Vacum and clean the vent hose and into the dryer as far as you get. If it's really bad, you may want to disassemble the dryer to clean it out completely.
If the air is coming out strong, then the problem may be with a worn heater element.
Please let me know if you would like more information. We will need the model number from behind the door to give you more specifics.