Strumming is an art in and of itself,especially combining open and fretted notes.Make sure to press down firmly and squarely on the middle of the fret.Using a thin or medium pick as opposed to a heavy pick will also help.The most important point of this question applys to all techniques...play steady and evenly,with an even amount of force on the upstroke and downstroke.If one note or several notes are more prominently heard than others,work on the technique until all notes are heard equally or as equally as possible.Lowering the "action" if possible may also help,but not to the point of fret buzz.So to review-finger the fretted note square in the middle,use a thin or medium pick,lower the action if possible,and strum with even force across all strings.Also,the lighter you strum the better chance all notes will be heard equally,hope this helps.

p.s.-Where you strum laterally also will make a difference.The closer you strum to the bridge I believe you have more control,this will make the tone thinner however.The closer you strum to the neck,the more open strings are heard.