Posted: 11/05/2017 at 13:28
It might be worth digging it out to inspect the roots, maybe the problems something local to where it is planted? And maybe give it a good trim back to see if that encourages new growth?
If there is no obvious reason why that one may have a problem such as its location or damage then I think I would be concerned it might be something that could spread to your other box plants, like box blight.
It might help if you post a close up of the affected leaves, I 'm not a pest and diseases expert but someone might recognise the symptoms?