ATO problem

I recently installed a Hydor ATO system in my sump. The sensor is located in the return chamber. Every day and sometimes twice a day (like as I type this} the alarm goes off and the water level in the return chamber is too high. What is causing this? I syphon out some water from my tank to get the level down in the return chamber but it just will happen again like 3:00 in the morning or the next afternoon, its becoming very frustrating. Please help.