This post can help you when your KDE Polkit Agent Helper misbehaves.
How would you know if you have this actual problem? That's easy!
If you try to take any action that would ask you for administrative permissions and the password window pops up and then disappears quickly, well, then you could have issues with your polkit-agent-helper.
sudo chmod +s /usr/lib/policykit-1/polkit-agent-helper-1
That should be enough for the permission window to work as expected again. :)
I hope this can help you out there!
See you next time! ;)