Hey Growmie,
I took a look at your diary and it looks like your young plant is suffering from a nitrogen lockout, which is normally related to your ph being off. I would first make sure your ph pen is calibrated and if that's fine try raising the ph to 6.3 - 6.5. Also, the daytime temp is kinda low I would try raising it to about 75°F which should also help. Hopefully, this was helpful, and remember ..." Happy Growing" and "Free your mind one puff at a time".