It seems like almost nobody manages their books on their computer. If nothing helps from here contact Amazon Client Service since you might have some issues with your wifi module.

Check under 'Manage your content and devices' (. Log off and back on your account under settings. Tip on the button Cloud and try downloading the book manually. Sever the wifi connection and sett it up again under settings. Set your device in flight mode for 10-15 seconds after which turn flight mode off. Hard Reset your device by holding the On/Off button 40 seconds or until you see a picture of a tree with a boy reading underneath it. If your device is compatible but the book still doesn't download try following: If the option is there you have to check if your device is listed otherwise it's incompatible. On the product page you will see something like ' Available for following devices' if this doesn't appear than the e-Book is compatible with all Kindle models. You should check if the Book is compatible with your device.

Kindle app sync devices