looks a bit lush so something impacting availability of Ca is a possibility too. Also, make sure pH has not drifted too low, as Ca will be one of the first issues seen.
the plant needs Ca at all times. so it won't be a 1-time addition to fertilization.. you'd need it each time. With soil, you could amend a bit more garden lime and maybe that eliminates need for something like calmag in addition to your normal nutes, but if it is necessary now, it'll be necessary until the end of the grow. if in soilless/hydro, this is much easier. I assuming your are somewhere around 1.3-1.6 EC (mix of nutes, not including tap water ppm) and 100ppm or more of that should be Ca. YMMV due to various local variables including your tap water. Good ballpark to start, though.