NFS service getting stopped automatically every 30days

We have one nfs server where around 20 RHEL clients access the /opt/applications folder. The nfs service goes down. Then users will get the error message as stale nfs file handle when they login. Need to find the root cause and rectify it to avoid this is future.

We are using RHEL6.8 on the nfs server and the clients.

This happens every month (around 28 - 30 days)

the nfs service need to started (1st rpcbind & then nfs) or the server need to be rebooted