Im having the same problem right now.

Was using a 20:20:20 mix, but at the same time, the plants were showing nute burn and N deficiency. I flushed the plants, went out and bought Urea to up the N levels, and stopped using the 20:20:20 mix.

I think yours have the same problem, marijuana doesnt need strong doeses of P or K untill the flowering stage, and would be much better to focus on N during veg.

to use the urea you dont have to crush it or mix it with the water, simply sprinkle a small handful (very small handful at first) on the surface of the soil. no need to put it in as to not disturb the roots. and water over the small rocks/pearls. they will dissolve and the nutrients will be absorbed into the soil. you should see the results in a couple days time.