Several things can cause large chlorine loss. Low CYA+full sun all day, algae bloom and the list could go on. How are you currently testing? Using a drop test OTO, "guess" strips or pool store results?

First thing to do would be to get a GOOD set of test results, preferrably from a titrating drop test kit such as the Taylor K-2006 and post those. Then we can help you figure out what's going on in your pool.