Your shrub's symptoms seem to point to a root related problem. I would suspect a root rot organism such as phytophthora may be killing the plants. This disease is more of a problem when soil stays saturated for days at a time. Good drainage is one method of avoiding the disease. In order to be sure what is causing the problem however, I suggest you contact your County Extension Office and see how they can help diagnose the problem. They may have you bring in some photos of the plants and may have a diagnostic service at the university lab. In that case you may need to dig up a plant that is almost completely dead (but still has some living branches) for sending to the state lab to see if they can identify them for you.
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