Answer: What you describe may simply be a natural aging process for your Daphne. Most do not live as long as yours has - not that this will be any comfort if your plant does succumb to old age. The acid-based fertilizer you use in the bed should not affect your Daphne in a negative way. Daphne's grow in the Pacific Northwest where soils are markedly acidic by nature and the plants thrive.
What you might try is a method used for revitalizing trees and shrubs that are in decline:
Bore holes in the ground in the fibrous root zone under and just beyond the drip line of the plant canopy. The holes should be 18 to 24 inches deep and 2 to 4 inches wide. Space the holes in rings around solitary plants, or, space them evenly between plants growing in beds. Space the holes one foot apart.
Then, fill the holes with sand, coarse bark or sphagnum peat moss. This will improve the aeration and permeability of the soil and help to restructure the soil.
Most important of all, water your shrubs during dry periods of spring and summer when rain is not frequent.
Don't prune at this point. Wait to see if treating the soil in the root zone will encourage healthy new growth. If so, prune as you normally do. Hope this helps revitalize your daphne.
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