Three years ago I planted bamboo as a hedge in my back yard. I prefer bamboo over standard cedar heding because bamboo looks a lot better. I have 13 Fargesia robusta planted, spaced approx. 3-5 feet apart. It may be worthwhile mentioning that all the bamboo is planted close to trees, however 10 of bamboos have been doing well with no issues, they are over six feet tall. 3 of the bamboo look like they have only grown 12" since I first planted them three years ago. What can I do to help the 3 that are not growing? They have had very few shoots and the space between them is not filling in.