If your chlorine level is at 0, then bacteria can be consuming the CYA and creating a high chlorine demand as well. If that is the case, then you need to add lots of chlorine to stop this process (i.e. kill off the bacteria) and it could take a lot of chlorine to do. When you first started this, the chlorine would read briefly for some hours, so that was good, but having it drop to zero is not. Just keep adding chlorinating liquid to the pool until the FC starts to hold. I wrote a timeline own experience with this problem in this post.