Ta test kit goes straight to clear from green?

[Eddles]

Hi all,

I have a Taylor FAS titrating test kit. I have a pool and a hot tub. I tested the TA of the pool, the test went from green to red as expected, all OK. When I tested the TA of the hot tub, it went straight from green to clear without going red at all. What does this mean?

I do admit the TA test bottles are slightly past their use-by date, but it seem to work OK for the pool?

Many thanks