I planted a row of these about 5 years ago. They were about 2-3 feet high and now about 6 feet, except for three of them. These three are about three feet high, so they did not grow much or not at all. My question is why are these three not growing? They are green like the others but never grew. Also what do you recommend as a growth stimulator or how should I amend the soil? My soil is more clay than sand if that helps at all.