Has there been any change like extending the outlet to a lower elevation? What is the height of the water when the siphon initiates? Has the recent flows been higher than in the past? For example, has there been significant flow over the spillway where before the siphon kept up with lake inflows and prevented flow over the spill way. The reason I ask is that it may be that in the past the siphon may not have worked even though it may have appeared to. I mean it may have flowed through the works while drawing too much air to initiate the siphon vacuum in which case the siphon may have worked more like an overflow pipe. A difference of a foot or so of pond elevation could be difference between the siphon piping working like a siphon or working like an overflow pipe.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers