Single tree in eleagnus hedge dying - why?

I have a hedge of seven standard eleagnus trees that were planted eight years ago.  They had continued to thrive until last Spring, one tree was visibly failing to throw out new leaves and died. Now this Spring the same is happening to the tree next to the dead one.  

I have had vine weevil in the garden at the back of the house where we have plants in containers only.  I water nematodes in to the containers to try to deal with them.

Is it likely that vine weevil are in the ground beneath these two trees and killing them?  Or any other suggestions?  

We had left the original dead tree in situ and are hiding it's bare branches with the growth from adjoining plants, now one of which is failing.

Any ideas as to what might help?

Many thanks