Can you see any other kind of files in NFS datastore?
It depends on iso file size?
Try to put there a little iso (less than 500 MB)
Another idea.
Did you apply this in vi3_server_config.pdf?....
NFS Volumes and Virtual Machine Delegate Users
If you are planning to create, configure, or administer virtual machines on an NFS-based datastore, you need to assign NFS access privileges to a special user, known as the delegate user.
By default, the delegate user for the ESX Server host is root. However, having root as the delegate user may not work for all NFS volumes. In some cases, to protect NFS volumes from unauthorized access, NFS administrators may export the volumes with the root squash option turned on. When root squash is on, the NFS server treats access by root as access by any unprivileged user and might refuse the ESX Server host access to virtual machine files stored on the NFS volume.
You can change the delegate user to a different identity through experimental ESX Server functionality.