DIAGNOSIS The dryer is using its timed cycle, not the "Auto Dry" sensing, because the sensor strips inside the drum are coated with fabric softener residue or lint. They can't detect moisture properly.
FIX You don't need any special cleaners. 1) Unplug the dryer. 2) Locate the two metal sensor strips. They look like thin, vertical bars running front-to-back on the inside top of the drum. 3) Use a soft cloth (an old t-shirt works great) dampened with a little rubbing alcohol or just warm water. Gently but firmly rub the strips until they shine and feel smooth, not slick or sticky. 4) Dry them completely with another cloth before plugging the dryer back in.
IF THAT DIDN'T WORK If cleaning doesn't fix it, you might have accidentally been using a timed dry cycle. Make sure you're selecting a cycle like "Cotton" and then pressing the "Sensor Dry" button until the desired dryness level (e.g., "More Dry," "Normal," "Less Dry") is lit up.
HEADS UP To prevent this, cut back on liquid fabric softener or use dryer sheets sparingly. Wipe down the sensors every few months as part of regular maintenance. Also, ensure you're not overloading the dryer—a packed drum prevents clothes from tumbling past the sensors.