Source viewer stays black

When i'm in edit mode and i move my mouse over the thumbnails of the imported clips. It doesn't appear in the source viewer. The screen stays black. When i double click the clip, i appears in the viewer, but i would like to see it appear before dubbelclicking just by moving the mouse over it. Thanks allready guys!

[Peter Chamberlain]

Check that 'show live media preview' is on in the viewer 3 dot option menu

That's the solution. It works. Thanks!