NFS mount hanging when mounted from fstab (Ubuntu 18)

I’m setting up an NFS mount on some Ubuntu 18 servers, following these instructions.

I have followed all of the instructions and it works when (on a remote) I run

mount /mnt/nfs_share

In the /etc/fstab file for the remote servers that are to be mounting the share, I have tried various options and ended up with the following line: /mnt/nfs_share nfs defaults,tcp,_netdev 0 0

When I boot, it does not mount.

Here’s the weird part. After booting, I check to see if it mounted and find out it didn’t. If I type mount /mnt/nfs_share, it hangs. If I type mount /mnt/nfs_share followed by umount /mnt/nfs_share followed by mount /mnt/nfs_share, it works fine.

Even weirder, if I type mount /mnt/nfs_share, it hangs. If I then remove it from /etc/fstab, and try again it says not found. Then if I re-add it and type mount /mnt/nfs_share, it works.

What am I missing?