Royal Canin Sensible - cat dry food

Strange, I took over my Persian cat from my friend in March 2024. He only eats dry food and, just like my Siberian, he really liked it at first. But with the change of coat in November/December, both of them became reluctant to eat it. Pusha sticks to her wet food and Carlo switched to Hairball, but he refuses it because it smells strongly of fish oil. I’m trying a different food after discussing it with the vet, as there might be an allergy???