moukie,
The situation you describe is consistent with the siphon breaking suction. This would be caused by air entering the siphon system and breaking the suction. Most installations are designed with a suction break that will stop the siphoning when the water level in the pond reaches a desired lowest level. In a design like this, there is a riser that will introduce air into the siphon. It is always open and has just the right amount of restriction to air flow to allow the siphon to charge automatically when the pond level rises to its highest designed level. This should be below the emergency overflow of the dam. Once charged the siphon should rapidly ramp up to a maximum discharge. While this is going on ... there is air being sucked into the siphon but it should be restricted enough that it does not break suction and stop the siphon. This design's goal is to break suction before your pond is emptied past a certain level. If you have this design, it would have a conduit open to the air and it could have a valve on it to adjust where it stops the siphon (lowest desired pond level). It must be restrictive to allow the siphon to get started. IOWs, it needs to restrictive enough to start before water goes over the emergency spillway. It must not be so restrictive as to lower your pond to an undesired lowest level.
First, verify you have a suction breaker conduit and that your installation is designed to operate as described above. If so, AND if the suction breaker conduit has a valve on it, AND that valve has been closed all this time, then you are probably losing suction either between the valve and the siphon or somewhere along the siphon itself.
