Firstly, check your Mg as others have suggested.
Secondly, I recall reading a Holmes-Farley article a while ago indicating that:
1. Alkalinity naturally fluctuates far more than Ca levels, so there's no need to panic when you see Alk drop but Ca remaining stable.
2. If you still see Alk dropping, add BOTH Alk + Ca in equal parts. Since the two are so closely interrelated dosing one independently of the other (the standard reaction) is almost always bound to lead to frustration. Do this with either a balanced 2-part additive or a Ca reactor.
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