I don't care how sophisticated their equipment may seem, but most pool stores test inaccurately, especially for the CYA test. I don't know whether it's training, lack of calibration, use of old reagents, or what, but you should use you own good Taylor K-2006 test kit for doing water chemistry tests. Nevertheless, they could be right about this one if the CYA didn't fully dissolve.

It can take several days to a week for the CYA to show up in the tests, depending on how quickly the CYA dissolves. It is slow to dissolve -- the fastest method has pure CYA hang in a sock or stocking over a return flow. Next best is in a skimmer sock or equivalent in the skimmer (careful -- you have to have alternate flow to the pump so you don't crack your skimmer). If you just add the pure CYA directly to the skimmer to get caught in the filter, then you should not backwash for a week and it will be slower to dissolve since the rate of water flow is quite slow when dispersed in the filter.

There is a liquid CYA called Instant Pool Water Conditioner that dilutes into the water right away, but it's about twice as expensive as pure CYA, though it also has the benefit of being roughly pH neutral.