Try setting a breakpoint in your code after the initialization and check the videomode and videodepth variables.
If you plan on running @ 1024x768x32/Fullscreen, make sure there is enough free memory (actually setting the desktop to 16 bit can help in this case, while it hurts when running in windowed mode). Next thing of course would be checking the A6 startup screen (what kind of video device was detected?) and updating your video card drivers.