Voices/Piano Playback Balance

When I playback a score consisting of 4 voices (SATB) and a piano, I can barely (if at all) hear the piano. When the voices are not sounding I can hear the piano fine but when the voices come in (or just 1 voice, for that matter) I can't hear the piano. Is that a known problem or am I doing something wrong? I can't find any playback options that allow me to select the relative volume of each part. Thanks for your help.