Some basic checks:
1. Verify network connectivity
- ran 'netdiag' at client and 'netdiag' & 'dcdiag' on DCs
2. Ensure that your policy are properly configured
- linked to the proper OU
- security filtering (Authenticated Users have Read & Apply Policy)
3. Check replication
- use 'repadmin /showreps' to ensure that there are no replication issue
- use 'gpotool' to verify that policies have been synchronized
Next,
4. Run 'gpresult /v' on client machine
-verify if GPO had been received and applied.