Here is some common error
1. IP tables/ firewall is ON stage
2. Name of the computer being allowed to share the directory failing to have its address resolved –DNS server can not see list in /etc/hosts file
3. Entering a user names as the anonymous user when the option requires UID number (such 505)
You can see shared directory following command
#showmount –e nfsserverip
It says device is busy
This problem occurs if there is a process holding the directory open. You can use lsof command to see any process holing the directory.
1.Wrong shared information.
2.Give the permission of the shared directory
3.Firewall or Iptables blocking NFS port it will display error like RPC failure from NFS client trying to mount NFS shares it may be because the firewall rules are not allowing request RPC and NFS port
root_squash and all_squash help server security for NFS