Some NFS server doesn't support locking file, therefore you had to mount the NFS device with the 'nolock' option set.
The system fstab will like this:
YourNFS:/ftp /mnt/nfsftp nfs rw,intr,rsize=2048,wsize=2048,nolock 0 0
or you can use the "mount" command like this:
mount -o llock,rw,vers=3 YourNFS:/ftp /mnt/nfsftp