I recently was called to look at a christmas tree farm with failing plants. The problem was related to planting too deep. The following is more information on this problem.
Many trees and shrubs are set too deeply from the time of planting, or they settle too deeply over time. A planting depth of only one-inch too deep can cause trouble. It is not uncommon to see trees planted as much as three or more inches too deep. Deep planting causes bark deterioration at the soil line, which will eventually kill the plant. It usually takes a few seasons for a newly planted tree or shrub to die from this. Various symptoms point to excessively deep planting. Some new growth may develop each spring, only to die-off during the stress of summer. Advanced symptoms of depth-related stress are cankers and deep cracking of the bark. A canker is an area of dead tissue on a woody stem. Some shallow cracking of bark is normal for many trees as the trunk grows. A tree may survive until fall but may not survive the winter because of an insufficient storage of food reserves caused by the damaged bark.
Information from "Common Abiotic Plant Problems", Home & Garden Mimeo # HG 86, University of Maryland.