Thanks to all who responded to my frantic posts over the last few weeks because of unmeasurable FC despite clear water, and CYA of 80-100. I was a skeptic, but after shocking for several days with gallons of bleach, I now have clear water, no slime, no algae. FC in the AM is 7-8, with OCLT of 0-1, and CC 0-0.5. My evening FC is 3.5-4. I am adding bleach every evening, based on numbers from the pool calculator, with a CYA of (probably) 35. The dosing is extremely accurate.
The big unknown is CYA. I use the TF-100 kit. After a 60% pool refill, I got CYA 30-35. My friendly local pool supplier got 60! I took a sample to Leslie's. The girl used a test trip and told me "0". I insisted she use a drop test. She then used a Rainbow kit with the black dot that pulls up on a little stick, and is read when the dot appears, rather than disappears. After multiple reads by her and me, we agreed on 35!
So, be very careful about who to believe on CYA tests; probably best to trust yourself! This is a highly subjective test, even when following Taylor Chemical instructions, or the extended instructions on this site. For me, the minimal and target FC levels are not much affected by CYA between 30-40, so I'm satisfied.
Thanks again. No one in my local pool community had any idea whatsoever on why I could not measure chlorine. Thank goodness for Google and TFP.