Another thought: have you checked lately to see if the correct device is listed as your default. Even if you only have one kindle, if you have the app on a phone or tablet it might have reverted to one of them as the default. I've set mine multiple times, but every now and then, for no reason I can figure, it switches to whatever is alphabetically first.
OR ... it could still *think* you have a device on your account that you no longer do, and that is showing as the default. So new stuff is supposed to go there but, as it doesn't exist, it just ends up on your account without being delivered to any particular device.