Yellowing leaves on hypericum

I have an "Orange berry" hypericum and the leaves are looking very yellow. A quick search online reveals it could be down to over/under watering or maybe some kind of aphids or thrip damage. Can't see any obvious culprits.

My first thought was it was possibly in need of feeding, so I gave it a seaweed feed a few weeks back but to no avail.
I have been giving a good soaking once a week, and the soil seems fine in terms of moisture. A few feet away from it is a Red Berry one which is looking fine as is the Hidcote one in another bed.

Any ideas/comments please ?
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