Well I cant see how big their pots are, but @ 7 weeks old, and with the light feeding (1/4 strength) you've been doing, I'd say that it's time to up the fertilizer dosage. The plant is getting bigger and hungrier. I dont know what kind of fertilizer you are using but you want a well rounded fert, something that contains Magnesium, and other micro elemnets in addition to N-P-K.
The other thing I would mention is that anytime you begin to have problems with MJ plant to make sure your soil PH is in an acceptable range between 6.3 and 7.0. Adding fertilizer to potted plants increases the acidity of the medium over time.
Were you able to get any help from the website with the pictures of plant deficiencies I linked to?
The more info you give the more accurate help you will recieve. What type of soil, type of fertilizer, lights, water, PH, pot size, temps, humidity, etc... all very important. Good luck.