Stem blight? Phytophthora root rot? Otherwise, are you watering enough, not too much; how's the soil ph? Any burrowing animals destroying the roots? Maybe you should ask over at Fruit and Orchards.

Could be as simple as a lack of yearly pruning on the shrubs that are not producing as much. I am far from an expert (as a matter of fact I am such a visual learner I would love to get my hands on an instructional dvd on how to prune blueberries) but it is crucial to yearly prune away some of the oldest wood at the end of winter. kt