There are a number of reasons a pool leaks -
1. the expansion and contraction of the tile grout during extreme heat and cold,
2. cracked or broken plumbing,
3. the sealant around returns and drains wear down,
4. crack in the interior surface of the pool or deterioration of the surface.
5. Equipment issues
Follow the guidelines in the article "Pool Leaks" for the bucket test. First determine the water loss. Where does the water loss stop? At the bottom of the tile line- could be the grout around the waterline tile or cracked tile. Or if the water loss stops at the bottom of the skimmer- resealing around skimmer or replacement may be required to take care of the issue. If a greater water loss it may indicate issues with the wall returns, main drain, or structural or surface crack. Having a qualified leak detection company follow up might be required.