It could be a lot of reason why, but from what ports you have shown me I did not see the NFS ports.

I. Please enter this command from client side:
1. rpcinfo -p IP_of_nfsserver
You should see a list that contain: portmapper, nfs, mountd, nlockmgr, status

That appears to be the problem. When I enter 'rpcinfo -p IP_of_my_NAS_server' I only get the following:
user:~$ rpcinfo -p serverip
program vers proto port
100000 3 tcp 111 portmapper
100000 2 udp 111 portmapper
user:~$
I only get the Portmapper, and not the nfs, mountd, nlockmgr or status.