We have a 20'x40'x10' in-ground with a vinyl liner (liner is 6 years old). We have treated for phosphates on a weekly basis and recently tested for ammonia that registered 3ppm.. Shocked the pool with 24lbs of burnout (granular) and removed the ammonia, however, we had no chlorine level within fifteen minutes and had more phosphates than previously. Had the water retested (within two hours of the first test) and put another 16lbs (granular) of shock in the pool after the test. Chlorine held for one hour at very high levels when tested with the test strips. Fourteen hours later and 3" of rain we are once again at 0 chlorine reading on the test strips. Pool filter is operating as usual and the sand was changed one year ago. Any suggestions?