Hi and welcome to YT
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Dogs generally only lose their appetite for any extended period if/when something is truly wrong. It seems like you're saying this is not just a day here or there, but more regular? If so, I'd really get her to the vet as something could definitely be wrong here since inappetance in dogs is a concerning sign.
If it's just random / here or there -- you may want to try sprinkling some Parmesan cheese on her food as this can really help entice dogs.