How can I find out what is holding a file or mount point, when `fuser` shows nothing?

I have a laptop that refuses to suspend. On the latest Ubuntu, the only way I can turn it off is to hold down the power button. The reason is because I have an nfs share that has gone offline, and something is still trying to commit data to it. I killed everything that was listed in sudo fuser, but it's still busy. Even the poweroff command gets stuck because it can't close the filesystem. How can I find out what is still holding it, and either kill or unmount it, and force a suspend, instead of having to manually, forcibly, shut down?