'Find and join a Wi-Fi network' window blank.

Hi all.

I am a bit puzzled by something I've just discovered while trying to help a neighbour with his wi-fi network.

If, on my own Mac, I click on the wi-fi icon in my menu bara drop-down appears with -currently- four wi-fi networks, including my own that I'm connected to.

But if I now click on 'Join other network...' at the foot of that drop-down it opens a dialog window entitled 'Find and join a Wi-Fi network. And, on this dialog box there is a button labelled 'Show Networks'.

I'm sure that when I've clicked that button in the past it has done what it says, shown me a list of the locally available wi-fi networks. ('Choose the Wi-Fi network you want to join from the list below.'

But now it is completely blank, as if there are no networks available at all. (And this while the four networks are appearing in the drop-down from the menu bar icon.

Has this dialog window list facility been erased on Mavericks or something?

I'm currently running OS X 10.9.1 on my late 2010 MacBook Pro.