Add hard disk - disk provisioning section grayed out with thin provision selected

[Alceryes]

ESXi 5.1 cluster - 4 physical hosts - connecting to NetApp via NFS for storage (and iSCSI for boot)

On random VM's we are experiencing a grayed out disk provisioning section when we try to add disks. There doesn't seem to be any rhyme or reason as to which ones we can and which ones we can't add thick provisioned disks. Any ideas would be greatly appreciated. None of the VM's have snapshots.

TIA!

[homerzzz]

My experience is Thin provisioned is the only option (why it is grayed out) with NFS on Netapp unless you have VAAI enabled.