I have a siphon drain that has not be siphoning for several months. Thinking it was a blocked inlet we hired a commercial dive team to come look. Before going into the water they blocked the outlet with a plug. Within 10 minutes of blocking the outlet the siphon kicked in and upon removing the plug siphoned as designed. We left the drain in siphon mode and when I returned the next day the pond was draining normally. After a heavy rain over the weekend the pond is two inches over the vent tube and no siphoning is occurring. Any ideas what might be happening?

stan