Hi there.
Check and ensure the following:
1) Album is exactly the same for all tracks
2) All tracks reside in the same physical folder
3) Disc # is the same if all the tracks are from a single disc (empty of a single disc, or the correct disc number for a multi-disc set)
4) Set Artist to the single artist you want to use (and clear Album Artist if it is the same as Artist) or for albums with various artists, set individual track artists in the Artist field, and either set Album Artist to the single entry you want to use for grouping the album, or leave it blank and MC will group such albums under the artist "(Multiple Artists)".
The easiest way to check sameness is to open the Action Window > Tag, select all the tracks, and examine the Album, Artist, Album Artist, Disc # tracks. If it says Varies, that means this field varies for the selected tracks. You can make changes there to unify to a single value.
Of note, the term "Stacks" in MC is for something different. You mean Grouping here.