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Watering would be my first question. That area looks like it might hold water a bit. H syriacus do not like a lot of water, and especially not a soggy root area.
They may need a bit of Iron supplement like Ironite. Do you fertilize or amend the soil for them? The proximity to the foundation and walkway may have the soil too far on the alkaline side for them also.
They seem to do best here for me with a bit of compost worked in around them in the spring with the addition of a little 8-8-8 granular fertilizer and some Ironite.