your problem could be down to a magnesium difficency i had this same peoblem and my plant wilted real bad and turned yellow
i went to a chemist and picked up a pot of epsom salts which was cheap about a quid 50
i then mixed half a teaspoon with 1 litre of hot water making sure it was disolved and then sprayed my leafs when it went luke warm and gave the soil a good feed with the solution too and you could see the results the next morning it perked right up
cheers all hope this helps ill attach a pic for ya to see
befour and after pics