This just means you haven't reached "breakpoint" chlorination yet. In other words, pools can need lots of extra chlorination/sanitation after. You haven't generated enough chlorine to meet that need yet. You can only test for "leftover" chlorine, so your pool is still needing every bit of chlorine being generated. After that point, it will build up the residual that you will test for. Simply leave your equipment running on high until that is achieved.