You say it hasn't filled up to the overflow in 20 yrs, that's kind of strange that you have never had a big rain event at some point in time that filled it up, even without adequate watershed. Is it holding at a certain elevation? as in when you do have some rain water that brings it up even just a little, will it stay there at that elevation till evaporation or something lowers it or does it go straight back down to a consistent elevation that it appears to stay at for the most part, or does it fluctuate quite a bit all the time?
As for the water loss amounts in a 24 hr period, I would and have stuck a yardstick in the water to check elevations daily, if you are not available on a daily bases to check it you can set up a game camera directed toward the yardstick and have it take a time lapse photo on a 24 hr basis. that information is pretty huge in establishing a pattern of water elevation rise or loss which will be valuable information for a professional like Teehjaeh55 to figure out whats going on. Id guess you have a gravel seem that is draining it down to a certain level and probably not allowing it to fill past said level, but that's just a guestimate because I have no idea what kind of soils you are dealing with at your site.
Good Luck!

All the really good ideas I've ever had came to me while I was milking a cow.