If NFS on the Server is enabled and clients are not able to successfully logon - here is some items to check to mounting nfs.
Please enter these command from client side:
1. rpcinfo -p IP_of_nfsserver
You should see a list that contain : portmapper, nfs, mountd,nlockmgr, status
2. showmount -e IP_of_nfsserver you should see a list of NFS shares
II. check if your ip address has sufficient right to enter chosen share
III. If above commands give a positive output, please use this mount options:
mount -t nfs -o tcp, rsize=1500, wsize=1500
IP_of_nfsserver:/share/SHARE_NAME
IV. It could be a firewall on client side, for check it out, please turn off firewall.