There are 2 possible causes for this.
First, if your screen is set to a low resolution (less than 1024×768) there is not enough room on the screen to display all the controls. A high resolution display is recommended for scanning because it lets you see more of the page at once. Use the Display Settings in your Control Panel to adjust your screen resolution.
The second cause for this problem is if the Display Fields setting on the Index tab is set too high. SimpleIndex will attempt to display the number of fields set here, even if they do not fit on the screen. Lower this number until all the controls you need are displayed.