Your pond sounds a lot like ours. We have sandy clay and sandy silt, I do think that some organic build up helps to fill in the areas between sand grains, but it takes a while. Mine used to fluctuate quite a bit, but it seems to be stabilizing.
I recently found that mine is holding water based mostly on springs fed by the water table. The Hydrologist who came out said I don't have enough watershed to fill it to where it was designed. He recommended either being happy with what I have, or consider a well.
