It's hard to say without seeing your calcium and alk readings over time to see how they are moving in relation to each other.
But, with calcium at the low end of the range, and I'm assuming that alk is low also (from your first post), it sounds like you just aren't supplementing enough. How much kalk are you using? Do you have a lot of calcification going on (lots of SPS, clams, etc.)? If you can increase the amount of kalk you drip, that should solve the problem. If you are maxed out on kalk (i.e. already dripping enough to replace all evap water) then you need to either add additional 2-part supplements or look into a calcium reactor.
Again, that's only if calcium and alk are dropping together. If one is dropping or rising in relation to the other, then you have other issues that need to be looked into.
BTW - be careful adding buffer AND kalk, because things can get out of whack pretty quick doing that. You should only add kalk once calcium and alk are in balance - if they are out of balance, correct with either buffer or turbo calcium FIRST, then when they are in balance, drip kalk to maintain it.