I opened my pool two weeks ago and my water is very clear, however, I can not get my FC to remain after shocking it about 10 times. My pool store told me to use 8 lbs. to shock that I had a chlorine block. I did so at 6PM that night, however the next morning, I had no FC.
I have an inground pool with 30, 000 gals. My Cyanuric acids are high so I stopped using and products with stabilizers two season ago. I am using a product called Poollife/Calcium Hypocholrite. The odd thing is I had a higher level of CYA last year and had no problem in maintaining FC levels. Listed below are my last test by the pool store. Their suggestion is to keep shocking , but after about 50 lbs of shock, I am not sure that will correct the problem. I did get some 6% liquid chlorine but I have no idea how much to use to shock .

FC .04
TC 1.69
CC 1.65
Tot Alkalinty (Adjusted) 81
pH 7.4
calcium hardness 484
CYA 94
copper 0
iron 0
tot disolved solids 1900
sat. index 0.1