Adding onto "Mancuniensis" answer"Mancuniensis" answer, herbs generally prefer a "loose" free draining type of soil. Think of where those plants grow natively, they don't like having their "feet" wet (as given in "Mancuniensis" answer"Mancuniensis" answer).
Do you have good drainage holes in your pots, containers? Just another factor to take into account.
If the "MiracleGro Potting Mix" has worked well for you, I would stick with that, or if you wanted to mix your own, you could try here for a good general purpose potting mix formulahere for a good general purpose potting mix formula.
