Iron def, really common,if you are giving the correct amount of nutrients and your PH is ok look for a Fe chelate (edhha o-o if possible because it works at wide ph range), you can spray it or give it trough roots, the 2nd one is obviusly slower, if you want to see a fast recovery spray them and if you see that they recover quick add a little amount 2 weeks after trough roots to enhace de availability trough the rest of the grow.
Always follow the instructions prior use and adjust the amount to the grade of deficiency you have usually the dosages are greater for foilar application and much fewer for root application so keep that in mind.