Do you happen to have any Xendesktop PVS or MCS virtual macghines on this volume? I have the same problem and Snapcenter cannot backup any nfs luns where I have MCS configured machines.
One thing to try..goto the VM's that are not working in the vcenter web client and try and try selecting individual VM's and see if it will let them select for Snapcenter protection. My MCS VM's won't let me select them. It could be because i have GRID cards added to those VM's but no one knows yet...still working through my case.