I had a similar problem and, after digging up around every pipe I could not see any water leaking out and I was losing a couple of inches a day..... Turned out it was being lost around the pool cleaner connection. I got some expoxy putty, that you need to knead together to activate, and pushed that, under water, around the connector. Cost me 4 euros and I have had no leak for 2 years.... Worth a try.
Richard in Crevillent - I have forgotten where I bought the expoxy….