chlorine level ?

[chem geek]

If you were adding this chlorine during the day, then it may have been getting broken down by sunlight. During direct noontime sun the chlorine half-life with a CYA of 75 ppm is around 7 hours. So at around 24 ppm FC, that's a loss rate of 12/7 = 1.7 ppm FC per hour (that's actually not the right calculation since the rate of loss declines as it approaches the "half" value of 12 ppm, but it makes the point -- the actual calculation shows that you lose almost 10%, or 2.4 ppm FC, in the first hour). That might explain the difficulty.

If you add it at night then any loss would have to be something else consuming the chlorine.

Richard