We had a similar issue and was notified by Google Support that it's a result of devices not being used in a "long time" meaning 3 months or greater as outline below
1.- The devices remained inactive for a while.
2.- Something get stuck from the device itself.
3.- Connectivity issues from your environment or student's environment such as network configuration, low-bandwidth, internet bouncing.
They suggest using the chrome recovery tool to have the device re-enrolled (even though it already shows enrolled in the Google Admin Console)