I noticed on two of my plants that when I transplanted too early from plastic cups to 5+ gallons they were slower growing than the ones I properly waited for. Ended up being that those plants were overwatered, and the soil took quite a long time to dry out. The small root systems stalled because they didn't need to keep reaching further to find water I think. Ended up not watering the bhut that went into the 14 gallon pot for about a month! During that time growth was very slow. Once it properly dried out a bit and the leaves started drooping, it got watered with 6-4-4 and in the past 3 weeks growth has exploded.
I agree that watering with a microbe brew/compost tea should have some good results for you. Maybe not immediately, but within a week or two you will notice a much healthier root system. One reason I love Promix and Tomato tone, they incorporate mycorrhizae already.