Nitrogen could be an issue, you say your transitioning those beauties for 12/12 (Flowering).. im assuming you backed off on feeding the plants nitrogen , or at least lowered there intake. Plants in Flower need phosphorus over nitrogen. Nitrogen for veg, Phosphorus for flower. Looks like maybe you quite giving nitrogen completely, thinking you are transitioning your plant. Making your plant stop nitrogen intake cold turkey. You could possibly be starving your baby by mistake, I may be wrong, you can find out by lettin that single plant veg a bit longer and hit her with a lil nitrogen and see what happens. Feed her nitrogen and leave her alone for a few hrs, if thats the issue, when you check in oN her, she should look more happy, she will have the thank you look. #2.. If you started her on Phosphorus already, even in a low dose to get her a taste for it, you could still maybe overdosing the plant on Phosphorus. Not real sure yet, what nutrients are you using, what soil, can you tell me everything you have fed your plants. The grow room temp, the humidity in the room when the lights are on as well as the reading while lights out. Ph can kinda be ignored for the most part as long as room temp, room humidity, c2 intake allong with well circulated air. Not just air from left to right but also from above blowing down on the top and air flow blowing from below up. Im not a know it all, i just love the green plants and watching them grow. i will help out any way i can. I remember when i first started. Remember, ask, listen, and dont be affraid to try things with your plants. Trial and error is the best teacher. I would deffinetly take that one plant out and vegg longer. Take her out, flower the others and if you get back to me i will give you alot of advice and give you instructions if ud like, and you can do to that plant my suggestions and you will see how much these plants will give you. I can instruct you on things to do, you will get just a small ammount less of a yeild from that one plant than all the others put together. Most important thing to always remember, practice makes perfect and also gives tons and tons of knowledge... Also, all plants are not the same, even some plants of the same exact strain grows differently, some take to nutes great and others get to hot and get burned, shocked, stunted, and catch all sorts of deficiencies. .