Some dogs have severe allergies or problems digesting chicken -- it is one of the most common meat ingredients to cause problems -- so that may be why you were told this. Can you go back and talk to the owners or their original vet to find out why you were told this? So many treats and foods have chicken.
The eating problem is very unlikely to be anything other than a common behaviour problem. This is a GREAT way for dogs to get lots of extra attention! :lol: Often a dog will not care to eat the first day or even days it settles into a new home, out of simple anxiety. But almost always, naturally the new owner fusses over the dog and tries to tempt it to eat by offering all sorts of new things. A new dog or puppy arriving to a new home is almost always a bit anxious and needy. It will be soliciting extra attention -- and if you reward the unwanted behaviour of the dog not eating, the dog immediately realises not eating brings rewards of extra attention. Many dogs (just like children!) are happy to go a bit hungry in order to create all this extra drama and get all this extra attention.
So: this is what you need to do as this WILL become a continuous problem -- as you have found -- no one thing satisfies her for more than a few days then she goes on 'strike'.
1) put the Royal Canin in a dish.
2) put it on the floor.
3) do not make a fuss over this or talk to her or even look at her
4) give her 15 minutes to eat during which time you TOTALLY ignore her
5) after 15 minutes lift the food without a word or a glance and if she hasn't eaten, that is her problem. Put the food away til the next SCHEDULED meal (which at her age should only be once or twice a day) and then repeat this process.
6) Do NOT give her treats in between the meals.
I can guarantee you she will be eating normally within days though she may hold out for even up to a couple of days. This is NOT a concern -- dogs are physically designed to go without food for several days and she will not starve herself. But when she realises the extra attention ploy no longer works, she will eat.
Once her eating stabilises, have a look at the feeding suggestions in the Caring for your Cavalier section (in the Library) as there are lots of suggestions on what you can add to make a meal more interesting. However you do not want to do this NOW until she is well established in eating normally, when you put the food down, within the time given. I would wait two months or so before trying to vary her diet by very much.
This kind of holidng out can be especially bad with singleton dogs as they've no competition for their food and they can be very manipulative.
I'd also have a readof the document on separation anxiety if she is overly dependent on you -- you don't want to encourage excessive neediness and it is good to work to train your dog to be happy when left on its own (eg to stay alone in a room while you do some work, or when you go out). For example you don;t want to make a big deal of fussing over her simply because you've emerged from the toilet no matter how happy she is to see you return :lol: -- or you reinforce her anxiious behaviour for worrying when you aren;t there. Similarly you enever want to fuss over dogs right when you come in -- ignore them til all their fussing settles and they quickly learn that the faster they stop fussing, the quicker they get the reward of your attention.