Disappearing toolbar icon
 

On a PC I support the 'mail recipient' icon was added to one of the toolbars.
This has been fine until recently It had gone. Now, when I rightclick on
the toolbar, select customize and add the icon, it is there. The moment you
click on it, not only does it not work but it disappaers again and the other
icons close the gap!
It's bizarre. Any ideas, please? (Outlook is the mail client and works
fine).