Just a thought.
By packing clay on the back side of the dam you are stopping the water at that point. The water can no longer flow through the dam so it begins to saturate the soil the main part of the dam is constructed of.
Saturated soil will not withstand the water pressure created by a pond of water and may even add to it.
If you pack as much clay on the back side as is in front it might work. Otherwise I would think it will eventually break.
Again, I don't know that the above is true, but it seems logical.
