Bill, are you saying the lake drops back to the original level before you raised your levee? If so, I would first suspect your clay borrow area for raising the dam as being too wet to be compacted properly if the area was from the normal pool area. Don't just check near your dam for moist areas, check several hundred yards beyond also.

Evaporation can lower the lake by an inch per day...well within the 24 inches per month range.

Does this have a drain pipe going through the base of the dam? What is your primary spillway?

I too had my levee raised 3 feet and my borrow area leaked into the hillside under the full pool level....the dam could be leaking also where the new clay ws added if it was not properly tied into the existing good clay.

Even at a 27 foot depth (same as my pond), you'd be surprised how little the PSI rating is.

It sounds like the dam and lake was well thought out and constructed, but the best clay in the world can't seal unless properly compacted...A Sheepsfoot compacter is best, sheepsfoot roller next best along with earthmovers....dozer tracks are some of the worst compactors.

I hope I gave you some new things to look for. And by all means POST PICS....We LOVE 'em!