kdenlive screen grab generating empty files

I am a new user and don't owen a camcorder yet but can record tv programs with kaffiene. I watched the tutorial on using screen grab with the record monitor and tryed to do the same thing but when I open the captureXXXX.ogv files with kaffiene or gexine all I get is a blank screen. can anyone explain how to properly do this in kdenlive? I used xvidcap to capture my work on the desktop and the resulting mpg files play fine and work with kaffiene, gxine and kdenlive as well but wont render full screen files due to the vidio settings, but I would reather use kdenlive to do this if I can get it to work.
any help would be appresheated!