My information about using multiple Time Machine drives remains surprisingly popular, even 4 years after posting it. I missed this one in the massive list of features, but Apple appears to have added more robust support for multiple drives in Mountain Lion!
Thanks to Macworld for highlighting it, in an interesting list of Mountain Lion system changes.
Andy's blog 
A NAS volume selected as a TM destination is not rotated to.
Rotating backups automatically works quite well in 10.8.2, however, I added a NAS volume as a third location for TM to backup to. The NAS volume works fine when manually control clicked within the TM preference window, but is ignored while the other two disks (USB disks plugged into a airport acting only as a smart switch (router disabled)) rotate one to the other during a typical work session. The NAS volume remains present within the TM pref window, but is skipped in order of rotation. Using a MacPro 4,1, OS 10.8.2, 1gig LAN.
Interesting. I have set up two NAS time machine volumes (simple shared folders from a mac mini connected to my TV), and it rotates between them quite happily.
I wonder whether it’s designed more for dual drive setups – or perhaps setups where only one drive is usually available (I rotate mine offsite).
There are 3 TM backup destinations. Two are managed by a TimeCapsule dumbed-down to a smart router (bridge mode); one is the TC’s 2nd internal drive and one is a USB drive plugged into the TC. These rotate well. The third destination is a Seagate BlackMagic 440 NAS set up per Seagate’s directions as a dedicated T M backup destination.
The T M rotates happily between the two volumes connected tot he T C, but will not automatically include the Seagate NAS. I can manually start and complete a T M backup to the Seagate NAS – T M sees it OK and accepted it as a valid destination. It will just not include it in the rotation sequence with the other two backup destination. Perhaps yours works so well because it is kept within Apple’s environment running off the mini – what do you think?
Yeah, that could do it. The shared drives on my mini appeared in the TM backup location options without any fiddling.
Perhaps look through your console logs (open Console.app, filter on “backupd”) to see if it’s simply having trouble authenticating during the automated backups.