These are my favourite plants and I have a lot throughout the garden, I would dig up the ones that are struggling, the water to these ones may not be penetrating the roots, also check the root system, it could be compacted, it’s what you can’t see that is the problem, by digging them up, checking everything, remove dead foliage, replant with a bit of fresh soil and water in with a mild solution of seasol you should be able to get them back looking healthy again.... best of luck.