Mind if I ask what API you're using? Is it from Overdrive's API or a third-party? Have an idea I want to explore if there's a decent API.
A niche thing which can affect stuff is some libraries only show their full catalog if you're logged in (and potentially which partner library you're a part of, if the library offers multiple). I don't remember if I just read this in their documentation or when scraping, found at least a few difference so logged in first