Dying plants in Hedge

A little bit of history first...

After moving to our new home about 4 years ago I cut down some dangerous and unsightly Leylandii on the boundary of the garden.  I dont know the age of the trees but they were significant trees - a good 30ft high and they had already been lopped - some of the trunks were a good 18 - 20 inch minimum in diameter at the base.  It was not practical to remove the stumps, so i used a stump grinder to take them below ground level.

Following this, i worked plenty of farmyard cow manure into the soil.

Almost 2 years ago i replanted the boundary with Green Privet to extend an existing hedge. The plants were ordered on the internet and were really small - 40cm- single stem plants in most cases.  They were planted in mid summer and despite watering i was really unsure how well they would take off.  However 2 years on i have been amazed as i now have a very dense hedge developing - in some cases over 5ft !!

Now the problem.

Last year in the middle of the hedge one of the plants that was growing healthily  suddenly died.

I noted it but did not replace it as the others were growing so well they were filling its place.  

This year the plant right next to it has suddenly started dying.  When i say suddenly- it is obvious now that it hasn't put on as much growth this year as the others so something must have been wrong for a couple of month, but it has flowered - but is now showing signs that it is dying.  All the leaves on the whole bush are clearly different from the rest of the hedge. 

Please can anyone suggest what i can do to identify what might be causing this as i don't want any more to go the same way.  

I can't see anything obvious above ground or on the plants, so I'm assuming it could be something below ground - possibly as a result of the leylandii stumps maybe??? 

Has anyone got any ideas on what i can do to confirm what has caused this?

I'm assuming i going to loose the dying plant - but again if anyone has any ideas on what i can do to save it then i would be grateful to hear - but i guess i need to establish the cause before knowing whether this is possible.

Thanks

Nick