The carbonate (soda ash) may be causing a localized large swing in pH in the area in which you are dosing, which would possibly cause your Ca to ppt out resulting in both the ppt you are seeing as well as the lower Ca. I would try adding it in a higher flow area, or perhaps use the S1 setting on your doser to add it more slowly. Personally, I mix my dKh up at about a 7:1 ratio of Bicarbonate to carbonate to pH balance it prior to addition, but you should still be adding your chems to a high flow area. Just one possible idea
Dan