Icons in system tray don’t display (sometimes)

I have a problem with a computer that does not display some of the icons in the system tray indicating that the application is running. This is not consistent, sometimes, at boot-up, only the virus scan icon appears, sometimes others. There is no apparent consistency. The speaker volume control icon, safe to remove hardware icon, printer icon, etc do not appear. I am satisfied that the various applications are actually running so I might as well have the shortcut available in the system tray. Anybody have any suggestions??