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

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!