What are you doing differently from one plant to the other? Some plants even if they are the same strain from the same breeder, will grow differently, just keep caring for her, sometimes its nothing you csn fix, look at my gorilla cookies diary, and the sweet gelato diary and notice differences between the plants, of same strain, same breeder, exact same conditions all micromanaged.
Sometimes is just not up to us, and that one looking sick might even be reacting to something the other still had to go thru and you might see the smaller one becoming the bigger one, so just keep going, you are doing a great job.
If everything is equal and they are different, its just about genetics.
Hope it helped, and if it did please do award me the grow question point, it does help out a lot for us growers answering questions free of charge ;)