What size pot are they in? I'd probably put them in something larger with some quality soil. They may be rootbound which can lead to ph issues or there could be some fert buildup in your soil causing problems.

If you are careful and use good soil they should be ok for a transplant.....better off then they are now I think.
Weedhound Reviewed by Weedhound on . Northern Lights 24 days into flowering and leaves yellowing Hi Folks Well, we have nearly made it on our first grow and we have two female Northern Lights plants that are about 3 and a half feet in height. Some basic info on our grow: Using a 250 W fluorescent light Growing in a general purpose garden compost from B&Q Feeding using a chem fertiliser - B&Q general puirpose plant food which has a NPK ration of 5-5-5 and contains trace minerals Have used organic bonemeal to supplement Ph levels Watering using boiled tap water Lights on a 12:12 Rating: 5