iTunes will group tracks into a common album if:
- all songs have the same Artist value and the same Album value, or
- all songs have the same Album Artist value and the same Album value, or
- all songs have the same Album value and the "Album is a compilation ..." flag is checked.
There do seem to be cases (starting from iTunes 12.0, as far as I recall) when grouping is not evaluated correctly even when one or more of these rules is satisfied. This can normally be addressed as follows:
- select all songs that should be part of a common album
- edit the Album field and insert a "dummy" character at the start - e.g., change "Dark Side of the Moon" to "xDark Side of the Moon"
- the songs should now group correctly
- repeat the edit operation to remove the dummy character - the album grouping should be retained.