Actually, your numbers sound right to me. 2 gallons of 6% should get you to 20ppm. But if you have CYA=100, you may consider that the shock level for CYA 100 to 200 is 25ppm. You are still getting a CC reading greater than zero which means you are still fighting something.

I would add 3 gallons then, to put you at around 23ppm, and see if it holds. Plus, you will need to maintain FC between 8 and 15 as long as your CYA is so high.

P.O.P.P: The most important ingredient--Pool Owner Patience and Persistence!

Yes, it's frustrating but high CYA forces one to commit to much higher FC levels than we normally think about.

You may want to consider dumping half your water and refilling to reduce your CYA level to 40 to 50--easier to maintain lower chlorine levels then.