hi susan. when you feed her, do you leave her food down all day, when she doesn't eat it--do you leave it there so she can eat it when she's ready?
If so, you could try just leaving the food down for 10-15 minutes then picking it up, then put it down again in a few hours and do the same thing, and then whether she eats or not, pick it up again and put it away. then put it back a few hours later. I don't know if that will help her gain weight but it has helped fussy eaters develop better appetites and better appreciation of their food. I don't know if you're doing this already or not but if not, you could try it. For it to work, you have to keep doing it even if she doesn't eat at all, for a few days if necessary. But usually they don't wait that long to start eating.
How does she look to you? Is her butt bony? are her ribs fairly prominent? When i met my first cavalier, i thought she might have cancer, she looked so thin, and i urged my daughter, the owner, to take her to the vet right away. My daughter took her and the vet said she was fine, that this breed is supposed to be trim. I took my Zack to a vet during a time when he was sick and the vet said he was underweight, by looking at him, not his actual weight, but every other vet i saw, which totalled about 4, said his weight was fine.
Apparently not all vets know about how thin cavaliers normally are unless they are overweight. I've heard that before. I certainly thought the dog was alarmingly thin when i saw my first one, but her vet said she was fine.
Your vet may be very familiar with cavaliers and drawing the right conclusion. I just want to point out that vets who are not familiar with them sometimes think they are too thin when they are normal and healthy. If you question this, maybe you can get a second opinion if you can get a referral to a vet in your area who is known to be familiar with cavaliers, if yours isn't. However, i'm in the US where cavaliers are far less common than where you are, and i would think your vet is probably familiar with the breed and knows what he's talking about.