providing you have'nt gone too heavy on the chicken manure, I dont think you have done anything wrong.
What you suspect regarding the roots being allowed to dry out in the garden centre is, I think the key to the problem.
I think all you can do now is keep up the watering and hope the other 15 pull through.
I'd remove any brown leaves, as they will not green up again also a mulch of bark chip around the base of the hedge will help retain moisture and suppress weed competition. Good Luck Tbird+