Do you have the reference solution for the checker? If not, there is no way to really validate if it's drifting out of calibration or not. As well, is that a new batch of checker reagents? Hanna have stated air exposure can eventually throw off testing accuracy, it's why the reagent container is so small for ~25 doses. I believe there are DIY recipes for making your own alk test reference from RHF using baking soda, might also be an option if you have a gram scale that reads decimals.
