I THINK, dont know, but would guess they arent drying out enough between waterings is the actual issue. Some nutes, like potassium i think, uptake better when the soil is dryer, some like phosphorous i think when its wetter. So if it doesnt dry out enough you dont uptake enough k so it builds up, and higher levels of k reduce the uptake of calcium and magnesium. So it could be that the soil isnt even defficient, just,like others said, over watered.