Bentonite works best if you can spread it and then cover it w/ soil before it has a chance to rehydrate/swell. The compression of the soil cover keeps it tighter when it swells. Same with bentonite-cored liners.

My idea would be to bore some holes below the dam (across the ravine if I'm reading correctly between the lines) to see where the water level rises to. You need to let the holes sit for a day or so to let the water level rise to equilibrium. If you have one or two holes with water higher than the others (may require a level to shoot the water's surface) you might be adjacent to the leak. Just because you don't see the seepage doesn't mean it isn't happening below the surface.

Another thing to try would be to pump down the pond and look for areas that dry up more rapidly than others.

Finding the leak can be tough. I've been involved in trouble shooting some leaking sewage lagoons that were lined with Bentomat liners w/o sucess. Sometimes you do get lucky though. The only constant to keep in mind is that water flows from high head to low head and troubleshoot accordingly.