Duck man - you are not providing enough information. Do you mean it drops two to three feet in a couple days? You have to get out there and measure accurately. Try setting a bucket of water in the pond filled to the same level as the pond. Leave it there until the pond drops a measurable amount and see the difference in level between the bucket and the pond level. Make sure the bucket is filled to the top so the water is not protected from the effects of the wind in drying the water surface. You then need to determine exactly how much water is coming in, what your surface area is, and calculate how much that influx should raise your water level over the same time period. If there is water coming in the bucket should not have more water in it than the pond. If the bucket loses more water than the pond then you are losing to evaporation. You probably also have some seepage.

Does it always drop to the same level and stop or does it continue dropping until a rain refills it?
During the six months when it dropped two feet was it all at once or gradual? You have to give full symptoms because their can be many physical causes for a leak and or seepage. You might have a good seal below a certain point but maybe a sand seam along the shore somewhere above a certain level. Might be your pond is very porous and it is just dropping back to ground level. Is it spring fed? Might be that the water is pushing back out some underwater spring into the ground table. You have to think of all these things and give enough info.

The way I search for leaks is to get in the water and walk on the bottom. This may not work depending on soil conditions and water depth. I have also found leaks by probing with poles when the water was very cold but that is a very slow and demanding process (and won't work on even a breezy day without some help keeping the boat still). I wouldn't even try either method until you are sure it is a leak and not seepage or evaporation.