I haven't cleaned our dryer vent in about 15 years so it was packed with lint. However I have a few suggestions. Add one extension at a time. Start with two snake the vent as far as they will go,pull out and add another extension and repeat adding another extension each time. This will allow you to remove small amounts of lint at a time and keep the brush from getting stuck. Also DO NOT flip the switch to reverse the drill when removing the brush. reversing the drill will cause the extensions to unscrew and you'll end up losing them inside the vent. I have first hand knowledge of this,so believe me. Just pull the brush out in a hand over hand motion.
Dean A. Reutter 
