Dryer vent clean out?

What is the best way to clean out my dryer vent pipe? I pull the flex pipe off the wall and exhaust is good. I reach in the wall and the pipe turns towards the roof. I go out side to what I think can only be the outlet but no air is coming out. This outlet is about 6 feet directly behind the dryer (a bedroom closet sets between the hall laundry room and the outside wall). I pull the flap up outside and insert a 4' stick straight in with no resistance (it goes straight in, does not turn up...)