I've not used a siphon, so just repeating what I've read here on PBF or other places.
The outlet has to be below the water level you are siphoning from. If the outlet is not low enough, it could siphon to a certain point to where the outlet was not low enough and stop.
Someone here on PBF had said to put a 45 degree fitting at the outlet to prevent air from following the top of the pipe up and breaking the siphon.
