Hi Tom,
The problem is likely that the NFS mount is already exported to this host, probably with a different NFS server name and most likely with a different configuration, i.e. one is mounted using the NFS IP address and the other using the NFS server name.
To confirm do one of the following:
1. From the Host object, select the DataStores view under Related Objects. Then right click the column headings, select Show/Hide Columns and ensure Devices is enabled. Then check and see if the NFS share is mounted using a different configuration than the others.
2. Using SSH, connect to the host and run "esxcli storage nfs list" - this will again output the NFS mounts and see which has a different configuration from the other hosts.
After you have identified the rogue connection and ensuring no VMs are running from it on this host, disconnect and then connect using the "Mount Datastore to Additional Hosts..." option.
Let us know if this helps.