top leafs truning yellow in flower??

ok im growing in coco, 2weeks in to flower and i have 2 plants: purp olger in a 1gal bag.. and the purps in a 3gal pot..the PH is 6.5...the prob is only these two plants, leafs near the tip of bud sites are truning yellow,up and down the leaf???...i just flushed then feed last night..to see if that whould help..does anyone know what this was caused by??......MEANGREEN..

got any pics?

not at the moment..i can get some tomarrow..but it looks like low lighting or N diff...but its on some top bud sites and lower one too..and i have a 1000watt MH so i know its not the light one of them is right under it....?

OK ITS A IRON DIFF ...GOLDFISH914 POSTED THIS LINK http://icmag.com/ic/showthread.php?t=11688 LOT OF GOOD INFO THANKS ....CHASE CLOSED...MEANGREEN..