2 things come to mind, 1 it's unlikely if you've kept the soil from being to wet, but whenever I've used soil, I let it dry completely, maybee even let it sit dry for a couple days, unless the plant says otherwise. 2 miracle grow is low on micro Nutrients, could be the problem.
Shit, I'm not sure of either of those possibilities. Let the soil dry out and see if thats the reason, and get some micro ferts. I really wish I could say more. You cant post a good pic, can you?