The things that I see most often is the start of an algae problem or one that lingers around for a long time. Chlorine goes looking to oxidize organics and if there is any algae in the pool that algae will consume your chlorine. The problem is, unless the algae is effectively dealt with the problem doesn’t go away. Each treatment for algae can be different. That’s why it’s important to talk to a pool professional and eradicate the algae outbreak. Even if you don’t see it, it may still be there. I hear all the time pool owners say they put a shock or two in their pool every day and the chlorine still won’t hold. Treat the pool for sufficiently for algae and the problem, in most cases, will go away.