It could be that you gave them too much fertilizer, I am guessing.
Whenever I buy an herb plant, the first thing I do is cut off the top and stick it into a Forsythe Pot to make another plant - and that's nothing but damp vermiculite - and they grow just fine. The original plant I put either in the raised garden bed with 'garden soil' or in a pot with 'potting soil'. I don't add any type of fertilizer. There is some of my compost and worm castings mixed in with my soils, but that's it.
