NIM NFS file system cannot unmount

Hello,

I have a script on my NIM Master that runs with crons and performs mksysb backups of all NIM Clients once a week.
There is a problem with one of the clients as it stucks and never finishes the mksysb backups.
I tried to unmount the NFS file system created by the NIM in order to perform the mksysb but i cannot do it.
I killed all processes from NIM Master and client about the nim operation but i still cannot unmount the NFS.
I even tried umount -f but i still get the message below:
i don't know what else to try. I searched through smitty the nfs filesystems for this server but it doesn't appear anywhere. I cannot understand why it still try to mount it.

Any ideas on that?
Thank you

Regards,
Stavros