Netatalk disk not shown in TimeMachine, manual connect via afp works
I've recently had the same experience.  Netatalk exported a volume with time machine set to yes, I could mount the volume, but it would not show up in the Time Machine preferences panel.

From the Finder I tried to create a new file/folder and realized that I didn't have write permissions.  Granting read/write permissions on the underlying volume on the file server then allowed Time Machine to find it.

The specifics of how to change permissions will vary but I changed ownership (chown) and permissions (chmod) on the mount point on the file server.