It could be a weird sync problem, like you have 2 screens, 1 physical and 1 buffer - and syncing switches between them, all the drawing is done onto the buffer screen, and they're swapped when you sync. This means that if you constantly sync without drawing to the screens, it just flicks between each screen - and it might be that one screen has your graphics, the other has nothing.
It might be a simple case of drawing your stuff twice, like a little For...Next loop with the sync inside would draw it on both screens, then flicking between them won't be a problem.
Van-B

Muhahahahaha.