Downloading sheet music
I just bought my subscription for MuseScore and I went to download a song with individual parts for my quartet. I downloaded it and only three of the parts showed up. It was a song for violin, viola, cello and bass. The viola part is the only part that did not show up and that frustrates me because I play viola in the quartet. I paid for the subscription and it's kind of annoying that the main reason I got it was for the viola part and that is the only part not showing up.
Comments
Try to use https://musescore.com/contact
If you use the free MuseScore software you can extract the parts yourself: https://musescore.org/en/handbook/parts
A Pro subscription is not necessary at all to get to parts, you can extract them all by yourself using the MuseScore desktop program.
As to why a certain part is missing from the download of a score from musescore.com as "PDF including parts":
If an author uploads a score with the parts already extracted, musescore.com leaves them as they are
If a score gets uploaded without extracted parts, musescore.com does it.
If now a score gets uploaded with only some parts extracted, I guess musescore.com leaves the score as is and does not extract the missing parts. So it seems here you'd need to complain to the uploader/author of that score.
But first verify my guess by downloading the MuseScore file and open that in the desktop program and check which parts are extracted there... we can't check as you didn't tell us which score it is.