Check the SMSTS.log for following:
Network Access Account Not Set
Resolution: Go into the Client Policy in SCCM and set a Network Access Account. It sometimes "disappears" even after everything has been working fine.
PXE CERT Not Set
Resolution: Check the PXE Certificate in the SCCM console. Verify that the Root CA is trusted. Alternatively, reset the expiration date.
Resolution: Ensure the FQDN is properly set.