Answer: It sounds as though those with the yellowing leaves and more extensive black spot are struggling - perhaps because of soil conditions. Although you'd think the soil is the same throughout the garden, some areas may not drain as well as others, or some areas might have competing roots from nearby trees. If the shrubs are small enough to handle easily, dig out those most affected with yellowing leaves and amend the soil to facilitate good drainage. Replant and water them in well.
Photinia is subject to black spot, especially if the leaves are wet overnight. Try to water the soil rather than the foliage (knowing that you can't control the rainfall) to avoid the spread of the disease. You can prune your plants in early spring, just as new growth begins. This will remove the old spotted leaves and, since the beauty of this shrub is in the bright red new growth, will encourage lots of healthy new growth.
Best wishes with your photinias!
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