root can't write to NFS mounted directory

[Mohan Singh]

I have some home directories that is mounted over NFS.

The users who log in via NIS can write to their respective home
directories, but root on the NFS client can't.

The NFS directories are exported via a NAS. I know it is not a
configuration issue on the NFS server, as it previously worked fine,
and the only change I have made recently is to wipe and install the
client.

What setting on the client allows root to write to NFS mounted directories?