A few times a month my wife insists that our 10 year old Amana dryer is not drying the clothes. She always uses maximum, timed heat (not automatic).
I've climbed on the roof and cleaned out the vent and have taken the dryer apart to clean out the lint. In both cases, there really wasn't that much lint!
I have only one elbow, all rigid metal exhaust tubing about 20 feet long.
I'm at wits end and am thinking of buying a new dryer. The kicker is, that usually it heats just fine. I would think that if it's a thermostat or the heating element, it would just stop working.
I have tried venting it straight out the back into the garage (eliminating any tubing). It happens so eratically, I'm not sure this is the problem.