Too little calcium in early flower?

Perhaps not calcium but magnesium. Since Cal-Mag is the likely fix it will cover both bases. Last year I ran nutes on Sundays and Cal-Mag on wednesdays, since the imbalance often swings like a sine wave as the chemistry of things runs it's course. If it's locking up, you will see it in the PH of the soil and the runoff after watering with pure 7PH RO water or a bottle of 'distilled' water from the store is even better, as it will counteract whatever is buffering you.