I have an avenue of yew trees planted six years. They are 4m tall now. Two trees in the row have been yellowing on the inside and the new growth looks fine. It leaves these two trees looking sparse and tired. I am told this is a known condition but have been unable to find out what it is.
If there is a draiange issue, try to address it, otherwise not much can be done.
It usually only affects one or two trees and yew is generally very robust.
Check also for the presence of scale insects.