that can be inconsistent or over/under watering, actual calcium deficiency, or a root problem. Smart money is to add some CalMag and check the soil Ph just to be sure. If watering was inconsistent enough at some point, than it can lock up the calcium in the soil. That's usually how people end up with blossom end rot in tomatoes. The cell walls break down, those spots are there for good, you will know for sure if it's fixed when new leaves are normal, and no new spots show up. Is there a drain in those buckets?