Randomly disappearing icons

Hello,

The shortcuts to log off, reboot and shut down in the kicker menu randomly disappear for me. I don't know how to restore them, so every once in a while I have to remove the whole menu and re-add it to the panel. Usually after some days, one of the icons disappears (e.g. shutdown) and eventually a second or third button will also disappear.

Why does this happen? And is there an easier way to restore my buttons, or can this be prevented completely?