Hey whiterabbit. Okay I got some questions for ya.

1. How old is your florecent light? Mabye you need to get a new florecent.I use a combo of soft White and sun light Red florecents. Also is your light close enough? 2 inch's is recomended.

2. Are you using any fertilizer? if so this could be your problem. Fertilization of a young plant can harm or burn the young roots and can stunt growth or cause yellowing and discoloration.

3. KNOW YOUR pH! Your pH in your soil could be off. It should be around 6.5,
7.0 is nutral and cannabis likes slightly more acidic soil. You can buy pH testers for under 20$

Hope some of this can help! Im sure you will get some better advice than mine! Good luck and GROWGOOD!!!!!!!!!!