Any bugs on the plants? If not, it what type of nutes are being used? Maybe a Mg deficiency..... https://www.rollitup.org/marijuana-plant-problems/157345-have-plant-problem-check-here.html
you may be right about the calcium def, but it generally caused by irregular waterings....... http://gardening.about.com/od/vegetablepatch/f/BlossomEndRot.htm