Also be sure to try and restart Tableau Server. That usually clears up a good number of things (as a restart does for most applications ).
If you restart the Tableau Server sometimes will restart the backgrounder process, but still sometimes it might fail to start. when it happened i have used the following process, like uninstall and install the tableau server which will fix this issue.
for me it worked. i know reinstalling the server is not good method but it will surely fix the problem.
So, we were able to resolve the issue - thanks everyone for your help.
The root of the problem was the firewall - while ports were opened for Tableau Server, ports 3730 and 3731 were NOT opened to support the communication to the worker node.
Once this was done, the backgrounders came online just fine.
EDIT: For clarification, the ports needed to be opened on the PRIMARY node.