Packed duct
Two cycles and the towels are still damp
The dryer runs hot but nothing dries, because moist air has nowhere to go. The duct behind the wall has silted up with lint over the years. This is also the failure that starts house fires: the element keeps cycling against trapped heat with a duct full of dry lint sitting right next to it.
We clean the full run to the outside, not just the lint trap.
