Not sure what to suggest really, Branston is on Royal Canin Junior Medium with a portion Forthglade Lamb menu (not the lifestage adult one, which he found too rich - but the Menu which is suitable for dogs from 3 months) and so far seems to be doing well on it.
It's rather odd really, as a personal preference I would normally feed wet hypoallergenic food, but Branston came on the Royal Canin and has done well on it, and had none of the problems that Minstrel did, before I changed him to a wet food, so although I would prefer him to be on a wet food, the old adage 'if it ain't broke, leave well alone' seems to apply here.
I think some dogs do seem to be a little fussy (have read before on the forum where they suddenly go off their food) have you checked that there is no medical reason, toothache for example, and also the hot weather lately may be something to do it?
Hope you find the solution soon.