The reason that NFS directory is non-accessible to root is likely "root_squash".
It assigns them the user ID for the user nfsnobody and prevents root users connected remotely from having root privileges. This prevents unauthorized alteration of files on the remote server.
To disable root_swash, set the
no_root_squash option. It turns off root squashing.
For example, to disable root_swash for /mnt/a. The line in /etc/exports can be: