I've noticed in a few of my pools that after adding CYA, or shocking with trichlor, a week later, the CYA still tests low. One of my coworkers (the only other one paying any attention to CYA) has also noticed the same thing. In one case it was a brand new pool in which we added 12 lb of CYA. I believe the pool was an 18x36 rectangle, so roughly 25,000 gallons. This should have brought the CYA up to about 60 ppm. When tested, it was about 25. It was tested with the tf-100, and about a week had passed since adding it in this case.
I don't know the specifics of some of the other pools, but I know that I have recently added significant amounts of CYA to pools, but when I tested them a week or two later, they were still pretty low. I believe I added 8 lb to one pool, and found it was only at 25 ppm today.
We have been getting a fair amount of rain, but the numbers I'm seeing suggest pretty significant dilution, similar to replacing 25-50% of the pool's water.
Any idea where the CYA is going?