Time Machine sometimes seems to have some issues verifying the previous backups when it's connected to a network drive.

This is the message you'll see:

This script fixes those backups, so you don't have to start from the beginning with a new backup set. It temporarily disables Time Machine, mounts the network share, verifies and repairs the sparsebundle file, fixes the plist file to mark it as valid, and starts Time Machine again.

You'll need to edit the three variables (USER, HOSTNAME, AFPSHARE) at the top of the script to your Time Machine network share settings.

Run the script as root, probably through sudo:

sudo ./fix_timemachine.sh

This script is almost entirely the work of Hoeve, with a few modifications to support volume names with spaces, and some comments.