Perhaps your dryer has 2 heating elements and one is bad. That would
give you noticable heat for small loads or no load, but it would be
insufficient for large loads.
If one heating element is bad, then here are some extremely crude
1) Get a replacement element mail order for $10-$20.
2) Get a replacement element from a local appliance parts store
3) Have it fixed by a repairman for $50-$100.
Even though this is a very simple repair, please don't attempt it if you
aren't proficient at electrical work.
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.