Yellow base of leafs/tops of plants.

I have no idea. I have 16 clones and 3 mothers and there is only one clone that is yellow at the base of all new growth. Thought about mg diffency but don't know why it would be on only one plant. ph ok, ppm ok, temps ok, all other plants doing great.

Any help? Here are pics. Another one has started to do the same thing. There is a comparison shot of a healthy one and a yellow one...

Sorry here are the pics

help! please + thanx guys