Don’t replace A/B/C separately- that was bad advice. Every box of reagents comes with ONE bottle of calibration solution, which is used 24-48 hours after you replace A/B/C.
if you do not follow that schedule and keep your old bottle of “A”, you will not be able to calibrate for the new box of reagent’s “A”. Therefor once you put in the next bottle (1st A from the new reagent box), you will un-calibrated, low accuracy Alk readings. Replace everything at once.
(Note this is talking about the complete reagent change after 2 months, not just the A replacement that occurs after 1 month)