Answer: It sounds like something is wrong with the roots. Could it possibly be that you are overwatering it? Soggy soil can cause the symptoms you described as can lack of water or some other root injury.
You might also examine the undersides of the leaves closely for black tar-like spots and black-to-clear winged insects. These are lace bugs which can build up on the older foliage of sage bushes, causing a decline. Insecticidal soaps applied in the early morning can help. Direct sprays from beneath the plant upward. Other natural options are sprays containing sabadilla or those containing pyrethrin plus rotenone.
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