Calcium and/or Magnesium def. Check PH and make sure you are not getting locked out. Add Epsom salts at 1 tsp/gallon or Cal-Mag if you have it to next feeding.
Sorry about the double post don't know what I'm doing yet new to the thread thing! I have to go buy a ph tester didn't know the ph was that important !
It's damn near impossible to diagnose an issue by only seeing 1 leaf, and a partial of another. I'm not even sure how old the plant is or how big it is.