Euron managed to find the part of the fleet that was headed for King’s Landing-which is where Euron already was, so it’s much easier to find that part of the fleet. There isn’t any evidence yet that the part of the fleet taking the Unsullied to Casterly Rock has been discovered yet
I thought the fleet Euron attacked was headed to Dorne to bring the Dornish Army back to besiege King’s Landing. The fleet going to Casterly Rock would have followed the same path with them until they got to Dorne, then it would have kept sailing to Casterly Rock.
Of course, maybe I am wrong. Maybe they skipped more than I thought and the fleet was already returning from Dorne when Euron attacked it. Is that how you saw it? If that’s true then it would appear Euron also destroyed pretty much the entire Dornish Army at sea.