One last thought, you don't mention the gallons per minute of your diverted stream. It takes an unbelievable amount of water to make a dent in a 9.5 acre pond. Are you sure you have the volume of water?
Lack of volume would still not explain the lack water getting back in the stream.
Goodness. Sheesh. Tough one.
Upon further reading, it appears that she's going right under the dam, or possibly around it. Sounds like the core wasn't deep enough. However, who knows how deep is deep enough? You may have run into major costs, had it been taken deeper, and tons and tons of ponds are never keyed and work fine, especially in areas of good dirt. A clay liner of the whole pond may seal it, keeping it from ever going below.
