I had the same problem at first, and I am sure this is your problem. Try putting a 45 degree elbow at the end of the pipe, pointed up. If you don't have a 45, use a 90. What is happening is the water is sliding down the pipe so fast it never creates the air lock, and therefore the suction needed to start the siphon. In other words, air is entering the end of the pipe and going back up. The elbow will keep this from happening.
