Dynamics for different parts

Hi,

I'm trying to get different dynamics for different parts, and it doesn't seem to be working. I'm working with two staves (the Grand Staff), and I often want to bring out one melody line or another,or have a quieter accompaniment. I assign the melody it's own part with it's own dynamics (set to "part") but it seems like the playback is inconsistent. For instance, I have a bass accompaniment, a melody, and a descant. I wanted to bring the melody out while having a quiet descant. I assigned each section (accompaniment, melody and descant) as it's own part, with it's own dynamic (set to "part", not "staff" or "system"). That didn't seem to work; one dynamic was influencing all three parts. I wasn't hearing the melody so I set it to FFF, and the rest of the parts to ppp. The result was all 3 parts playing at ppp. Then I changed the accompanying dynamic to "staff" and that let the treble staff suddenly play both part 1(the melody) and part 2 (the descant) at FFF (even though the descant, part 2, should have been ppp). So very inconsistent! How can I get different parts to play at different dynamics? (Ideally, part one in treble would be able to have a different dynamic than part one in the bass, but I'm not sure that's possible.)

Thanks for your help!