problem with red robin hedge

we have 3 very very large trees bordering one corner of our garden (2 sycamore, and one maple style tree). our problem is we have planted a red robin around it but it keeps dying. we watered it regular etc. we need a hedge there because we need some screening from road. hedge will essentially be planted beside roots of trees, we have red robin surrounding front andd one side of large garden (1/2 acre approx) just need something for that corner. can you advise please