ALT monitor or video problem

I can’t get a ALT monitor to expand my H-size settings to the end of the screen, when on 800 X 600 X 32 bpp. It works fine on 640 X
480 X 32 bpp. The screen does expand to 46.8 / 75 Hz. That is the max shown on the screen. Is their a way to readjust this?

Thanks
Richard Bolha
hvl@one.net