If you are down to 25% from full in Jan you have a major leak, you will get some seapage a lot of times in a new pond till the clay seals itself if not compacted enough, but you have much more then a compaction problem, you should be able to see a decent stream of water surfacing somewhere.
How big is your BOW? it would take a pretty decent stream to lower a pond that much. evaporation shouldn't be a huge factor this time of yr.

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