You likely have what we call a chlorine demand, which there are two ways to solve:

1) The SLAM method. I know next to nothing about this, except that it’s a long, drawn-out process that I don’t have time for (as a tech who visits accounts once a week).

2) The second is to consistently double your shocks day to day. Start with a standard dose, say 2lbs for 20,000, and retest the after 24 hours. If there’s still no chlorine, add 4lbs. Retest after another 24 hours and if there’s still no chlorine you add 8lbs. If you still have no chlorine after a third 24 hour interval, add 16lbs, and so on until you establish a free chlorine level. I once had to put a 64lb dose of shock in a pool to get it to 1 ppm chlorine the following day.