Skimmia looking very unwell

Hello,

I planted two skimmias about six years ago. They are in near identical situations about ten feet apart, planted in full shade at the base of a north facing fence. I feed them both ocassionally with ericaceous liquid feed, along with a few other skimmias, azaleas and pieris I have. Soil is constantly slightly damp due to the position in the shade.

One of them is doing fine, but the other started to struggle about a year ago. I have attached pics of both for comparison. The poorly one started to go yellow and wilt on a few stems on one side initially, so i removed the bad bits. In the end i removed about a third of the plant in spring in an attempt to stop it spreading further across the plant. This seemed to work and new growth started to appear. sadly the new growth hasn't come to much and the whole plant is now looking very sick. The leaves have turned yellow, have lost their gloss and have also turned red underneath. Anyone have any suggestions of whether it's salvagable, and if so how?