I know i say this in almost every thread, but it was a simple fix to my big problem.. i had a plant that was 4 weeks old and it was 3x smaller than the other two planted at the same time.. try adding some nitrogen to that little one, get something organic that wont burn like fish emulsion ( i have 5-1-1 ) or bat guano ( theres others too ) but the nitrogen really helps speed up growth and takes the yellow out and makes them look green and healthy again..

btw, im not a pro grower.. just adding my .02 cents, never hurts to try..