Have you confirmed on Apple's DEP site that the iPad is configured for the correct MDM server? You may need to move it to another MDM server. Here is our process for setting up DEP, for your reference: EPM MDM DEP Setup and configuration
Please note that the certificate process must be completed on the core server. Once you move the device to a new MDM server, wipe the device and attempt enrollment again.
If the device is enrolled to the correct server, you may be running into an issue with applying the inventory data to the database. Just after enrollment, check event viewer on the core server and look for any errors or warnings for the "LANDesk Inventory Server". The warnings and errors in event viewer should match the time stamps of files in %ldms_home%ldscan\errorscan.
Please make sure that you have all of the MDM ports open on your core server.
You may want to enroll the device using the Ivanti agent app on the app store and see if that works. That should help narrow down an issue with DEP or communication.
I hope that helps.