Flow begins seal little pipe, syphone starts vacuum breaks at the bottom of pipe level by air rushing in.
If flow equals vacuum seal, syphone will not break. Decrease flow. look at the link for size matching to bed size.
there are some cheats, some are quite smart, some defeat part of the idea, of air pullen in as water pulled out.
try this out: http://youtu.be/4hHRe3KJfoY
I just set mine up and had the same problem.I started by allowing water to enter faster and after that didn't help I slowed the amount of water entering the grow bed and then it has worked perfectly
Siphons failing to start, or stop properly... are almost always related to flow...
The affnan siphon allows for a greater range of flow... and siphon reliability...