Well, you have no chlorine in your pool again. There is only one answer....more chlorine.....as much as it takes.
It is descriptive of ammonia which consumes chlorine very fast and then, when it (ammonia) is all gone by adding chlorine, your chlorine residual will start to show up. Even if it's not ammonia, the fix is still the same.....more chlorine.
Tell us some history. Did you close the pool this winter? Did you add chlorine over the winter?