Even if it's a Ca deficiency, it's probably not because you're not adding enough. Have you checked runoff PH and EC? Personally, I think runoff PH is only meaningful if it comes out WAY out of wack, like 4- or 7.5+, but the EC can get pretty high. If you're giving them 2-300 ppm
CalMag in a 900 ppm feed, you could end up with a 2-3000 ppm in your coco, a grand of which might just be Ca. That much
calmag also comes along with more N, which they may not be wanting, depending on your base.
I tend to think that its easy to go overboard on Ca and Mg for a lot of growers in coco, just because of all the stuff they read online about it. In other words, it's easy to piss off a happy plant because we convince ourselves its not happy. I run the same thing in coco that I run in HT and DWC. I back off the Ca and N half way through flower, to make room for a bit more of everything else. Smooth sailing every time.