There are several potential reasons you may be experiencing difficulties uploading or downloading files over 20GB:

1. Check your internet connection for speed and stability.
2. Ensure your device has enough RAM to handle large file uploads & downloads.
3. Delete unused files off your device & OneDrive account to free up storage.
4. Check your firewall to confirm it does not limit large file transfers.
5. Try uploading & downloading files during non-peak hours.

If these steps do not resolve the issue, please contact your IT support manager.