We have a 3.5 year old Kenmore gas dryer which is taking much longer (2 to 3 hours vs. 30 to 40 minutes) to dry recently. If you run it empty, the temperature is hot, it seems normal. With any clothes in it, however, the dryer rarely heats.
I don't know how these things work, but I'm guessing that the vibrations caused by the full load are affecting a thermostat?
I verified the vents are clear - the airflow at the exhaust port outside is comparable in intensity to that coming out of the back of the dryer. I vacuumed out the vent line just in case, and there was nothing in it.
DB