Suggest you contact your local extension service for a much more focused response to your questions. If they are young and recently planted (ie. just before you moved in) the could be suffering from transplant shock and not establishing properly. water is usually the problem with a limited root system. If old plants there may be a disease present, slowly moving from one to another. Check for rodent damage feeding on the bark near the soil line. Environmental stress can take several years to actually cause a decline and death of plants.
