In general, most man-made dams leak. The big questions are; (1) Is the amount the structure is leaking acceptable, ie. is the reservoir drawing down unacceptably. (2) Is the water exiting the backside of the dike clear. If it is muddy in color you may be loosing fill which could lead to an eventual breach.
I generally do not become concerned when I see structures leaking "slightly" and the water is clear. You may however want to plant some water loveing plants (grasses not trees) in the seep area to control erosion.
The higher the dam the more "head" pressure on bottom of the structure, and the leak may get worse.