Choisia death in sections

Hi guys : )
I have two large choisia quite distant from each other that r dying in sections . A section will yellow and die , I'll cut that beach out at the base and a month or two later another section will do the same . It's been happening since late last winter and it's only a matter of time until they're gone completely. They are in beds around a big lawn . less exposed choisia along sheltered paths r fine . helllppp ! :cry3

 

Is there any chance of a picture or two please wozwoz. I don't think it's anything to do with the winter months. I have Choisya 'Sundance' and it was buried for two weeks under a snow drift but suffered no ill effects apart from the weight of the snow had pushed it onto it's side.

How old are they? That may be a factor. Could they possibly need feeding? Unless there is some sort of visible disease there is little else that comes to mind.