Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
Your books are automatically delivered to the kindle it is purchased from.
While you can't change the default for that, the books are still availalbe on the other,
you just need to go into the CLOUD section of library/home screen, rather than the Device library.
When you select an item in the cloud library (which has all of your prior purchases on the account available), it prompts it to download to that device for local storage.