Dryer is hot but not drying?

Recently my Whirlpool electric dryer has pretty much stopped working. It would turn on, spin, get hot, but the clothes will still be wet after hours of drying. There would be noticeable moisture at the bottom of the drum, and also the lint collector stopped collecting lint. I suspected a blockage in the exhaust vent, so I replaced the tube and removed a ball of lint from the portion of vent that extends through the wall. This vent goes vertical through the wall. Does anyone know what is causing this?