There are several things you can try. First you might want to confirm you have a leak by putting a bucket of water in the pool, perhaps on steps, with the same water level as the pool. If you are losing water through evaporation only the bucket will go down at the same rate as the pool. A leak will cause the pool to go down more quickly.
Another trick is to measure the rate that the pool loses water at various water depths. The water loss will slow down as you approach the level of the leak.
Next is a through visual inspection. Get a face mask a snorkle and swim around looking closely at everything. With a vinyl liner leaks will often cause small craters in the liner as they wash out whatever is behind the liner. When you find something that looks questionable try a small squirt of food coloring and see if it gets sucked out through the possible hole. When working with dye you want the water as calm as possible, pump off, swim gently.
Leaks can be a real drag. Good Luck!
