One of the ways to enroll devices in Intune is to hybrid join them to Azure AD and then use group policy to autoenroll them to Intune.
In one of my projects I did exactly that, but for around half of devices autoenrollment failed. The first thing to check when autoenrollment fails is the DeviceManagement-Enterprise-Diagnostics-Provider Event Log.
There were two errors reported there:
Auto MDM Enroll: Device Credential (0x0), Failed (Unknown Win32 Error code: 0x80180026)
Auto MDM Enroll: Device Credential (0x0), Failed (Unknown Win32 Error code: 0xcaa90014)
As it turned out the problem was that for these Windows 10 devices old SCCM agent was running, after uninstalling the SCCM agent devices shortly appeared in Intune (autoenroll scheduled task tries to enroll to Intune every five minutes).