This Knowledgebase article provides information about why network shares are unavailable on the passive server in a Neverfail server pair protected by Neverfail for Fileserver.
Availability of the network shares on a server depends on the lanmanserver service's status. If the lanmanserver is not running, the shares are not visible in Explorer or cannot be accessed.
A Neverfail server pair at all times has an active server serving clients' requests and a passive server hidden from the network. On the passive server, the Protected Application services are stopped (to prevent synchronization issues). When the Protected Application is Fileserver, the lanmanserver is one of the protected services and as such, is stopped on the passive therefore preventing access of the shares on this server.