I don't have a solution, but the things I'd check for or try would be to issue:
in order to see if that gave me any further information. I'd also check the system log for this, which you likely have to determine that your NFS mount is causing the problem. There are complaints about this, mainly in the forms where the NFS mount is inaccessible at the time of shutdown. For instance the connection or the server went to sleep and so when the shutdown tried to umount, it had a delay or didn't work by never finishing. In some cases people say it just took a lot longer to shutdown, and in some cases, people say it never completes and stays on. The next thing would be to research whether or not the NFS mount could be configured in some manner where it could be umounted without a lengthy delay or umounted without pause or complaint.
A final thing to do would be create a script in place of poweroff and redirect the likely, already existing symbolic link for poweroff to point to your script. And in your script see if you can force that umount to occur no matter what the case, or at least perform the umount as part of your script and then deal with the case of unable to umount if the network was detached.