The overflow box has two chambers...one in the tank and one outside the tank.
In scenario "A", the tank-side chamber is full (level with the tank water surface). The outside chamber stays low. The return pump continues to pump water into the tank leading to overflow of the tank. This situation, BTW
, is one that you should have some sort of protection against (float switch), because it is what occurs should the siphon every clogs or stops (snail in the pipe syndrome).
In scenario "B", the tank-side chamber is lower than the water level of the tank, but the outside chamber fills with water. This would have the effect of slowing or stopping the siphon action, but is a different problem. As a rule, the outside chamber should always have a lower water level than the tank-side chamber. If the outside chamber fills up with water, it slows the siphon and moves you quickly to scenario "A".
Where to attack:
A: the siphon is not strong enough for some reason. It isn't pulling enough water. There may be air trapped in the siphon...or the tube may simply not be large enough to move the volume of water that the return pump is adding.
B: The outflow (to the sump) is not flowing fast enough to keep up with the siphon. The outflow hose could be airlocked or not large enough. Use a VERY large pipe for your outflow. Larger than the diametre of the hole in the bottom of the overflow box. I use plastic sump hose available at Home Depot.