I run a multi monitor setup and an indispensible tool I use is UltraMon. Recently, not sure when, I noticed that my Smart Taskbar was inexplicably missing from my additional monitor. I had all the correct settings as shown here on the left.
To let UltraMon create a new Smart Taskbar that spans multiple screens do the following:
1. Close UltraMon (right click the UltraMon icon and click Close) .
2. Open the registry editor by typing ‘Regedit’ in the Run command window.
3. Delete the registry key 'HKEY_CURRENT_USER\Software\Realtime Soft\UltraMon\Smart Taskbar'.
4. Restart UltraMon.
NOTE: It worked for me but … deleting registry keys is a risky business. Follow this advice at your own risk.