Help: I have Yellow leaves

[lenmyers]

I have five plants. Four are in great shape, but one has developed yellow leaves that seem are moving up the plant. The plants that are outdoors and receive the same amount of water, nutrients and sunshine.
Since my other four plants are thriving under the same conditions, I am at a loss to understand what could be happening to the fifth Plant.
Until the leaves began yellowing, this was my best plant.
I would appreciate any thoughts or feedback.
Thanks in advance

 

[Bushbandit]

Are you able to post a photo ?

 

[Farmville]

What is your grow media you are using? What kind of feeding schedule do you have? Be specific if you can. Pictures would help a little more as well. Also what strains are they and what seedbank did you use? What size pot are they in and how old are they? Or are they in the ground? Are all five outside? What kind of foliage is around the plants some are toxic to other plants and make the one closest sick.