You test the water. Chlorine looks fine. pH looks fine. The pool still looks like weak milk. That is common, and it is usually mechanical.
A dirty filter cannot polish the water. A pump on too short a cycle does not turn the volume over. Fine dust, pollen or dead algae can stay suspended even when sanitiser reads well. After a big swim day the pool can cloud from sunscreen and body oils. Brushing, running the filter longer and cleaning the media fixes more of these than adding another bucket of chemical.
Fibreglass interiors are smoother, so they do not harbour as much muck as a rough concrete finish, but they are not immune. If a new pool is cloudy, call us. After-sales support is part of how we work. We would rather come and look than have you guess from a forum.