this girl gets the same feed as the girl next to ...

Magnesium deficiency? If yellowing is less interveinal over time, then i'd say a slight N-def. Epsom salt can work too, if not too high in Sulfur already. Cal-mag is fine, if you are not too high on Ca already. May want to calculate ppm from your guaranteed analysis labels on your nutes. make an informed decision. consider recent behaviour and events that may not have been included here... in the end make sure you understand the decision you choose to take, don't blindly follow what i specifically say.