you have many things to watch and also post back with. Such as...

Where are the brown spots all over? Just on the ends of the leaves?
Miracle grow......Not your best option for MJ , some may argue that. In all reality it is probably the miracle grow stunting the growth as the soil components are usually not organic and have ferts for plants other than MJ.

What is your feeding schedule? When did it start and at what concentration?
I ask this because you probably have been feeding a bit too early for such young plants. Right about the 3 and 1/2 week mark you should start feeding in soil but your soil was feeding from day one. Do some reading and reconsider your options. Try to get some pics up if you can and think about transplanting before its too late.