Hi Annemariestory,I have numerous box's in my garden and some of them go yellow as well thanks to my dog but others for no reason at all,they are all treated the same.It could be box blight?,Monty Don had to dig all his border up and burn,heartbreaking.What I would do is buy plants from Homebase/b&q/Range something like £6 for 8 plants and pot on.I like them because they look good in pots all year round,see my photos just put on,regards ,kerryvw.