I recently had a new liner installed last month. The installer put in a new seal strip around the fiberglass steps. The pool now leaks 3-4 inches of water in a day. From what I can see, they strip is the hard plastic strip which is screwed into the concrete wall over the liner and a beauty strip. Should they have used a gasket between the liner and the hard plastic strip?
We tend to use the original seal strips whenever possible, but sometimes replacement is needed. The installer may need to recheck the screws, and a bead of caulk /underwater silicone(such as 801) can be added to help prevent leaks. A gasket is not usually necessary, but we often add a bead of 801 during our installs. Link to 801
The water loss issue could also be something completely unrelated to the step seals so some more troubleshooting may be in order.