There are several issues going on here.
The first is that you should not EVER add a pH buffer to maintain a specific pH. That is not the best way to even raise pH, and is probably driving the alkalinity too high. What is the alkalinity now?
High alkalinity will cause precipitation of calcium carbonate, depressing calcium.
As to the calcium supplement, how much are you adding? You may need more than you have been adding. If you are trying to boost calcium from unusually low levels, you typically need to add more than the bottle recommended dose, as that is a maintenance dose, not a correction dose.