problem with copper beech hedge

I planted 140 copperbeech hedges in 2007 but 110 didnt take, garden centre replaced them and I replanted them in dec08, but again 65 didnt take, planting them all the same way good base with compost, 1ft apart and zigzag formation against a post and rail fence with wire. I’m very dissapointed,and I’ve no idea what to do next. I had planted all this hedge to encourage the local wild life and I was willing to wait the years it would take to grow and fill out, but its very expensive and labour intensive to keep replacing dead plants. It seems odd that some of the plants take but others dont. Should i try a different type of plant, any suggestions?