I'd give them a shot of Sequestrene iron tonic, then use an ericaceous feed designed specifically for acid loving plants - Miracle Gro make one for Azaleas and Camellias, that'll be fine for the Skimmia too. But the sequestrene will sort out the discoloured leaves.

Bamboo is right. I did this recently with a Skimmia I had rescued that was in poor shape and within a relatively short space of time I'm getting lots of lovely green shoots.

How often should I feed my skimma. it sbloom now.