I have been buying the AquaMate brand of CYA (supposedly 100%) for 5 years. Every year it is a struggle to get my CYA up. I add more and more but the CYA barely moves. By the end of the summer it is maybe 50. When the cover comes off, it's back to virtually zero. I was convinced that I had that CYA eating bacteria. I was just about to buy an ammonia test when I read on here that some people have had trouble with certain brands.
So, I bought some HTH CYA, put in 4 lbs per pool calculator and VOILA! I'm at 60! The 4 lbs bottle was a dollar more at WalMart than the other stuff, but I don't have to put in 3 bottles.
So happy. I'm thinking there's a good chance it will disappear over the winter again, but that's ok if I can get it back up in the spring.