AsNoriuanswered grow question 3 years ago Calcium lock out or lack ??? Hard to tell, I see small pots, so they could overgrew , in this case root problems could start too.
People still make it in solo cups, so you should too. If you overdry plants too much they burn cal out of leaves, if you water on wet you screw up pH and then calcium deficiency or lock out kicks in, it's most common deficiency, but why it happened to little info.
Good luck !