yellowish hellebores

OK - so I know they must be deficient in something, but any clues as to what? They're in a bed that's a little shady (although the ones in pot next door are fine) that picks up quite a few yew needles. They're not really yellow yellow, but certainly not as healthy a green as they should be. Any ideas?

 

Tangle, I would just give them a feed with something like Phostrogen or MiracleGro.