Your leak will be around the cinderblocks or other additives. There should be no foreign matter, including rocks, in the dam. Rocks in a dam are a real no-no. I would tear out everything and replace it with loam; no silt added. The dam soil will have to be well packed at each stage. Dam builders often use a sheepsfoot roller to properly compact the newly added soils. Rocks, cinder blocks, electrical boxes and other stuff like that is most likely the reason the original dam failed.
