Chlorine lock
Also called: over-stabilization, over-stabilized water.
Chlorine lock is a pool-store term for water where the chlorine reads fine but does nothing. Chlorine lock does not exist: Trouble Free Pool calls it a myth, and what a store calls chlorine lock is almost always over-stabilized water, meaning the cyanuric acid (CYA) is too high.
Why it matters
High CYA binds up so much chlorine that a normal free chlorine reading can't sanitize, and algae gets an opening. No amount of shock breaks a lock that doesn't exist. The answer is less CYA, and CYA only leaves the water by dilution: partially drain and refill, or use a reverse-osmosis service if you can't drain. Skip the CYA reducer bottle. It usually got high from trichlor or dichlor, so switch to plain liquid chlorine or a salt system. Get an accurate CYA number from a drop kit before you drain anything.
In numbers
Replacing about half the water lowers CYA by about half. Recheck and repeat if it is still high.