Site Expand/Collapse

When I click on the expand/collapse icon of the Site Files dialogue box I no
longer get a view of both the remote and local files. What happens is that
the whole thing disappears. If I try to re-activate it a rectangular dotted
shape moves from the task bar to the screen and disappears.

I can get the Site Files box back by closing Dreamweaver MX and opening it
again but can only ever view one set of files at a time.

Can anyone explain what is going on and how to put it right?

Ken