Something isn't adding up. From what I'm seeing in your pictures, and what you're saying about the amount of rain that you are getting, I would expect that pond to fill up easily.
I agree that land with trees in it will hold more water then pasture will, but that's mostly on flat to gently sloping land. Your valley is too steep for that to be a significant issue.
The most obvious concern is if the dam is leaking? I think that would be obvious by looking at the other side, which I'm sure you've done many times.
Did you dig a keyway when building the dam?
Second concern, is there a leak that's off to the side of the dam? Tree roots are famous for this. But gopher holes, or whatever else digs in the ground can create a path for water to follow too. Probably the most common, and hardest to find is a crack in the ground that's caused by rock, gravel, sand or just some random thing that happens to the soil.
I would be looking along the edge of the water for anything at all that might suggest water is getting into the soil there. Also looking all over the downward part of the valley, past the dam might show a wet area that shouldn't be there. I've seen water leaks from pipes that showed up a hundred yards away from where the pipe was leaking. Odd things happen underground!!!!!