To investigate why an iBook isn’t installed, you should go to the specific iBook and search for the device within the scope where the app isn’t installed yet. Over there you can see the status of the app and device.
To be sure what’s wrong, you can perform a media check in the last column of the device in scope. This will tell you exactly what is wrong.
If you're unsure what to do further, please contact customer support through a ticket within ZMS.