There is something working on taking hold in your pool. Bring your chlorine up to shock levels and keep it there until the chlorine holds overnight. You must be consistant. Everytime you let the chlorine drop down, the culprit is trying to take hold. The only way to get out of this pattern is to hold the pool at shock levels until they hold overnight. Try to test 3 x a day, if that isn't possible add more bleach to hold over till you test at least 2x a day - but make sure you test at night after the sun goes down, and before the sun gets on your pool, this way you will be able to tell when the chlorine is holding.