Standard Poodle won’t eat

Hello all, my family and I were lucky enough to adopt a 3 year old standard poodle (Maya). I found her on Instagram being rehomed by her breeder after her original owners had her for 3 years. I asked if there were any issues and her breeder had said that she was underweight, barely ate or liked anything, and had submissive peeing issues. I’m thinking there was some sort of abuse or neglect. Previous owner kept no medical records it was weird. We have a 6 month old standard poodle (Emma) who is awesome loves to eat, high energy, and all of the above. We took Emma to meet Maya. They hit it off and get along for the most part (some jealousy issues but we spread the love). We have been on this weight gain journey with Maya, but it is really hard because she’ll love something for 2 days and on the 3rd she doesn’t anymore. She’ll either eat breakfast and lunch no dinner, one day or not eat breakfast but eat lunch and dinner or any of those combinations. We have discovered a lot of foods she likes but can’t seem to be consistent with it She likes canned food (Evangers), raw burger patties, some treats not all, will dabble in some kibble but hardly ever. Anyone have any ideas or guidance on how we can get her to gain some weight or at least be consistent with meal times. She’s currently a soft 40 pounds. Thanks in advance sorry for the long post.