Why is this happening

If you just put a layer of MiracleGro on top the seed's roots were probably through it in the first day or two. Roots, especially when first starting go straight down and can easily be 5-10 times longer than the plant above ground. In the future get a piece of pipe or roll paper into a tube about 4-6" in diameter (10-15cm). Place the tube in the center of your pot and fill with the mild potting soil and fill around the outside with hotter stuff like Ocean Forest. Pull out the tube and you now have mild soil in the middle for the young plant. When it matures and sends roots out sideways then it taps into the more nutrient soil.