Try these steps one at a time until the book(s) download:
1. Turn on the Kindle's wireless, and then do a Sync (menu option or icon)
2. Look in your Cloud/Archives/All tab and see if it's there - if so click on it, and it will download to the Home screen.
3. Go to your Content and Devices page on Amazon (selection under the right top menu) and click on the Actions button next to the title and select send to Kindle (make sure you select the correct Kindle if you have more than one registered to your account), then turn on your Kindle's wireless and do a Sync and Check for New Items.
4. Reboot your Kindle (hold power button in the ON position for 30 seconds), then start at step 1.
5. Try to send it to your Kindle via USB cable. Go to your Content and Devices page on Amazon (selection under the right top menu) find the book and click on the Actions button [...] to the left of the book. Then click on Download and Transfer via USB and select your Kindle from the list of devices registered to your account.
6. Contact Customer Support.
Click on the "Help" link at the bottom of this webpage. Scroll down, click on "Need more help?", then click on "Contact Us".
I'd suggest you use the Chat or Call Me/Phone option. Do NOT use email.