I agree with those guys, mostly about being careful with the nutes. Your plant is behind schedule, so just because it's 3 weeks old, doesn't mean that you feed it like a 3 week old. It needs fed like a 7-10 day old, which is zero fert, in that soil you're using. You can worry about feeding it when it's about as tall, and wide, as the pot it's in, which'll tell you that the roots have likely occupied most of the space in the pot, and have the soil nearly depleted. More sunlight is what you need, if you want to speed things up, right now.
The downfall with MG soils, is that they can retain too much water, as-is, outta the bag. Add some perlite, and they work great. They can work good outta the bag too, especially the potting soil(as mentioned), as long as you have your watering habits fine-tuned. The hard part, I've found, is keeping the edges moist, while not leaving the center-mass soaked, especially with small plants in big pots. When the plants get bigger, that becomes a non-issue, because they wick so much water.