In our business, we have seen it all, in terms of how and why an appliance is not functioning as it should.
First and foremost, the most common reason your dryer is not drying properly is that your vent (and perhaps your blower housing inside the dryer) is clogged with lint and cannot push the air through. That is a huge fire hazard (over 15,000 dryer fires per year), and as you can see on our home page with our Youtube videos, we think it’s a big deal. If it is over 6 feet in length, has elbows, goes up to the roof or down under the house, it should be professionally cleaned/inspected every year. More often if it is a significant distance.
If your dryer takes too long on sensor dry, the sensor itself may be coated and needs to be cleaned to properly sense the dryness of the load. The use of dryer sheets is probably the culprit here. Dryer sheets leave a residue that inhibits the ability of the sensor to do its job. A professional can take care of this for you.
Your clothes may be too wet going into the dryer. If your washer is not spinning the clothes out well enough, it stands to reason that it will take significantly longer to dry the load. The load should not be “wet,” just damp when they come out of the machine.
Your timer is off. It is not staying on as long as it should to dry the load. And your timer may be the old-style knob type or a control panel or PC board telling the unit what to do.
You are washing/drying loads that are simply too big for the unit to handle. I know that laundry is a huge chore, and the fewer loads the better. However, jamming those clothes in will just put unnecessary stress on the washer and dryer, which will put stress on the components and shorten the life of the appliances. Not the best for all concerned. Definitely wash only full loads, but items need to have a little room to move. An added bonus is that they will come out cleaner.
A little yearly TLC, a few changes in your habits, and you may see a significant improvement in your dry times.