At the start of the NJ pool season I was having trouble holding a Free Chlorine reading (FC of nearly zero and TC of 3).  To solve I was advised by the pool store to add 90lbs of Calcium Hypochlorite shock (~60%).   I've done that and after my chlorine returned to a normal level there is still a differential (free chlorine if 2.8 and TC of 5).   Pool looks great, PH is 7.4, alk is 107, CYA is 91, phosphate of 1188 and Calcium Hardness is 338.   I've got a 31000 gallon plaster pool and am beyond frustrated that I can't fix the issue.  Please help!