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.