Well, if you have all the fittings air tight, you are probably sucking air though the ball valve and it is couteracting the siphon. I ran into the same thing when I was doing some preliminary tests. You need to have some tubing connected to the ball valve via a hose barb fitting and keep the tube full of fluid after the level drops below your ball valve. This way gravity can still pull the rest of the liquid out through dip tube via siphon. If you end up needing to pull the hose out of the liquid for any reason, it's nice to have a siphon stopper on there to keep the tube full of liquid so that siphon will continue when you're ready. Here's a pic of the tube I fill my fermentors with.