It's usually because you've got
mismatching data in the "Album Artist" tag. I always ignore this one and leave it blank, and use the "Artist" tag instead. For some reason the default Android music player always thinks that because some albums have different artists, e.g. compilation albums, they're different albums. There's some information about this feature/issue/bug here...
Issue 2397 - android - Music player "Artist" classification should use "Album Artist" tag - Android Open Source Project - Issue Tracker - Google Project Hosting
i.e.
It's not a bug; it's an undocumented feature!
You can still have different artists in the "Artist" tags, as long as the "Album" tag matches, it won't separate them out. Also might be worth trying different music player app.