It would be impossible for CYA to go from 100 to 80 without an exchange of 20% of the pool water. More likely variances n the testing (skill, lighting ect.). Of the tests it's the one that takes some practice to master.
So, we have proven that there are organics in the water, so it's time to SLAM.
Not, sure if you have read the article yet, but here it is again.
We’ve all had to deal with a green pool. Clear algae from your pool with a SLAM! Read here to use shock level and maintain to clear your pool.
www.troublefreepool.com
Leave the salt water generator off, no solid chlorine products and turn the pump back on. Your goal is to keep the FC at 24 or slightly above that (not massively) all the time. Test the FC every couple of hours (or as often as you can) and if below 24, add to bring it back u p. With your liquid additions you have been basically doing this most of the time. Algae attached ot pool surfaces can develop a biofilm that protects it from the chlorine, so brushing once a day breaks this up.