Can anyone help me with my box that I'm trying to grow a hedge out of.

Some of my past plants have had this problem and I replaced them.
Now a Plant that is three years old has had it happen to it.

What is it, is there a cure and how can I stop it happening to the rest of
the plants?

Here is a picture of the plant.

http://img129.imageshack.us/img129/1587/01050001mz4.jpg

Kev