Diagnosis
The moisture sensor bars inside the drum are coated with fabric softener residue or lint. They can't read the dampness correctly.
How to Fix It
Unplug the dryer. Locate the two metal sensor bars running along the inside top of the drum. Scrub them thoroughly with a Scotch-Brite pad and rubbing alcohol. Wipe dry. Run a test load without any softener sheets or liquid softener.
If That Didn't Work
Check the external venting. A kinked or clogged 4-inch duct will severely reduce airflow. The dryer can't move moist air out, so clothes never dry.
Heads Up
Using too much liquid fabric softener directly causes this. Use pods or add softener to the washer's dispenser, not directly on clothes going into the dryer.