Diagnosis
Inadequate heat. The drum is tumbling, but the air isn't hot enough to evaporate the moisture. The most frequent causes are a clogged vent (again), a failed heating element, or a cycling thermostat that's stuck open.
How to Fix It
Run the 'Check Vent' diagnostic first—it's built in. If that passes, you need to check the heating system. Unplug the dryer. The heating element is housed behind the back panel or sometimes accessible from the bottom. You can visually inspect it for breaks (a obvious gap in the coiled wire) and test it for continuity with a multimeter. Also test the high-limit thermostat and the cycling thermostat on the same housing for continuity.
If That Didn't Work
If the element and thermostats test fine, the issue might be with one of the thermal fuses (there's more than one) that are part of the safety circuit for the heater. If one is blown, the control board won't send power to the element at all. You'll need your multimeter to check them for continuity.
Heads Up
Before you tear into the heating assembly, make 100% sure your vent isn't restricted. A clogged vent is the #1 reason for poor drying performance, and it's much easier to fix than replacing internal parts.