Questions could be asked about dryness, wind rock from some of the gales you might have had. We had to take out a Robbinia after a gale loosened the roots. Not had one since, as they get too rampant if not tamed. The soil in one place might be less appetising for a given shrub than another place in the garden. One can only guess.

Wind in the UK tends to come from the SW, so I would also assume it is the wind causing defoliation. It should right itself over summer. Can you plant another plant to act as a shelter? They do get large as Dorjac has just said...which reminds me to move mine at some point!

Red Robin is a very vigorous shrub once it gets going. It can be treated by hard pruning. Round a leisure centre outdoor pool I used to go to, they were planted for privacy and were probably 16 feet tall.

No one garden maintains a consitant climate throughout, so it is highly likely that one side receives frost whereas the other doesn't, or at least degrees of difference. Above commenst equally apply, and as suggested in the later post, without photos, it is very difficult to apportion an accurate diagnosis.
That being said, why worry. The summer will sort them out, probably very nicely, and I would suggest that concern need only be raised towards the end of the growing season should things not improve.
Remember, plants have a natural affinity to survive, so whatever problems it has encountered, it will try to counteract them.
The one advantage us humans have, is that we can react more positively than what might at first seem to be natural.