Answer:
1 - My hypothesis would be that the cat doesn't want to eat the Tuna because of the flavour.
2 - Experiment : Feeding the cat with tuna flavoured with chicken soup.
Explanation:
1 - The cat has been eating tuna for some period of time. Let's say that he migth be tired of it, and when Isabel fed him with flavoured food, it felt the change. So, now, he doesn`t settle, he knows what he wants and won-t eat anything else.
2- I would make some chicken soup, or something juicy with meat, and then go to the cat bowl and mix the tuna with the liquid. If he eats it, you can be certain that he is just beeing selective.