There is a balance that needs to be meet between three things. The amount of air in the top of the cap that has to be drawn out cant be too large or it has trouble starting. Needs to be able to start draining and pull out that little bit of air. Too little air and it has trouble stopping. Obviously the top needs to be level too. Also, the amount of pipe after the drain leaves the tank affects it. Needs enough back pressure to create turbulence to create the suction to remove that air. Third is the rate of water filling tank. You have to find the range where the water is entering fast enough to start the siphon, and slow enough that it will break the siphon. It seems stupid simple, but it really is more difficult than you might imagine. I used a glass mason jar as a top half to watch the water flow while i got the drain working correctly. Then I timed the fill/drain cycle to get it where I need it. Hope that helps
There are many different types, configurations and proven designs available online for bell siphons. I just learned a newer more simple design today by a fellow aquapon, however, my first attempt is still working fine with a few hiccups now and then. My first thought on yours was bigger 'stand pipe', try 1" instead of 3/4". Just a thought. Good Luck, though it sounds like you might already be good!