I also think you have an air leak that is stopping the siphon. Several years ago, I was worried that a 2" valve would not let in enough air to stop a 8" siphon. It is absolutely not a problem. Stops it quickly so... An air leak will definitely stop a siphon.
If you can borrow a gas water pump, enough water hose to run from the pump to the downhill end of the pipe, and one of those clog busters (see link below), you can ensure the downhill side of the pipe is full (might even help find if there are any holes in pipe). If there are no air links, it'll work well.
What you do is make sure pipe is airtight, put pump intake line in water at pond, connect water hose to pump on one end and clog buster on other, insert clog buster into downhill end, start up pump, let run for long enough to fill pipe (watch for pipe sinking, listen for water running through into pond, etc.), and then shut off pump. Clog buster will deflate, water pressure pushes it out of pipe, siphon started.
I've used another brand clog buster in 2" pvc repeatedly with great luck. I have the 4-6" one shown below; but have not tried it yet.
http://www.amazon.com/Oatey-33442-4-6-Cl...8691&sr=8-2
