If trees grew on the pond dam they can provide channels for leakage if they rot out.

If the pond is dry the best bet might be get a pond builder, clean out the pond and rebuild the dam at the same time. There is a difference in a dozer operator and a pond builder. Anyone who can push dirt with a dozer is a dozer operator. A pond builder with experience has the knowledge to build a pond that will not leak. That knowledge is vastly different for different areas and different soils and clays.

My suggestion would be to find a local pond builder and have him look at the situation for advice.

You will find a lot of good information here. Click on "Ask The Boss Forum". It is the equivalent of AgTalk for ponds.
https://www.pondboss.com/Home#mce_temp_url#