Cant get calcium levels high enough

So, all my levels are good at this point in my 72 gallon bow...live
rock is doing well, as is my cleanup crew, clownfish, and PJ
cardinal. I have one zoanthe in there, and it's thriving nicely as
well...gotten the algae issue more or less controlled, and we're
rolling right along. I've been adding a 2 part solution alkalinity/
calcium buffer in an effort to prep the tank for some more corals, and
the alkalinity is right where I want it...but for some reason the
calcium seems stuck at about a 300, where it should be in the 500
range...is there anything more that I should be doing? I've been
adding the solution about every other day for around 2 weeks now, and
the calcium just seems to be stalled out...

Mitch