low chlorine

[mark r]

cannot get chlorine to last, using far more chlorine than in past, all other levels are correct, including stabilizer -- chlorine drops to nothing overnight

[chem geek]

Probably a nascent algae bloom not yet visible. Shock with chlorine at the appropriate FC for your CYA level. See this link and the one on shocking that it links to.

If there is > 1 ppm FC loss overnight, then this is most likely fighting algae growth. If the loss is only during the day, then it's too low a CYA level though it sounds like you've ruled that out.