Ok, I got so excited about my reply that I didn't read what everybody posted - so forgive me if somebody already said this.
A few years ago, my granny bought a new dryer, because she figured the old one was taking too long to dry clothes. She bought the exact same model I had just purchased. Her clothes still took too long (way longer than mine), so we knew it wasn't the dryer. We were befuddled.
Come to find out, her dryer vent is in the middle of her house, going straight up out the roof. It had been built 20 years prior, and had never been cleaned out professionally.
Now that she's had it cleaned out professionally, we wonder how long that old dryer would have lasted????
in other words, her house vent was her problem. Not her dryer. I'd check the vent hose in the back, too....