From your comment that they can't be sent to your current Scribe, I would suspect that they are a format that can't be sent to an eInk Kindle though I've seen several children's books that say they are compatible with the Colorsoft but not the B&W eInk Kindles.
Take a look to see if any of the books in question have a line in their information that reads:
Quote:
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This title is only available on select devices and the latest version of the Kindle app. Please refer to the supported devices list before purchase. Available on these devices
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Where the
Available on these devices is a link that pops up a list of devices/apps. See attached image for an example though that list has not been updated in a while.