What is your nitrate and alkalinity levels? It might be easier to archive a low N, P and Alk system compared to rising P and keeping HA away. For example I keep my alk at 7.3dKH, P is mostly zero with Hanna Checker and N is around 0.5ppm.
Right now, if you add more P, it will just cause HA to grow more. It is the reason why your P is at 0, all P you add is being taken up by the HA.