I had the issue where I have a set of Albums that are a part of a series but all have a different subtitle so I didn't want to put them together as a single album and lose that subtitle. One of the albums had the same "Artist" for the album, so it was grouped by the artist and not with the others under the "Compilations" section. This is caused as the Artist is the same on all songs despite the checkbox for Compilation being checked as it is only evaluated if things above it in the list are different. iTunes does not have anything I could find to change how it evaluates the sort order. So I started to figure out a work around for the situation. This behavior is 100% by design with Apple's coding.
As my workaround to resolve this issue of Compilations (can have issues in views other than "Grid View", as most workarounds might fix one issue to cause another depending on application use) you can add a space to the end of one of the artist field for a single song provided you want the album to group under the Compilations section (it only works for the Song Artist not any of the other sort fields). Doing so will force there to be different artists (will appear to be the same because the space is invisible to the eye). This will mean as iTunes evaluates the "Song Artist" they are no longer all the same so under the rules it must no move to the Compilations section.
One negative effect that you should consider before doing this is, you will see what appears to be duplicate artists, which they technically are, in views where you are looking at the artist as well as in artist auto-fill fields such as fixing artist information for an album or song.
This extra space also explains why you might not see things grouped as you expect them to be, an extra space or different spelling is usually the reason why things do not come together as a single album (when a compilation that checkbox (dropdown must be checked or turned to yes). A simple way to check this, select more then one of your albums with the issue and then "Get Info". If they have different Artist then it is fine for it to say "mixed" however if the "Album" or "Album Artist" say "Mixed" iTunes will not group them together as they are different in the string comparison. To resolve this these two fields must be the same. This is why when changing the "Album Artist" to a different one like the ZZZ and then back again fixed the issue as you make the strings the same.
As a Software Engineer, I would say that it would be nice if they would simply modify the sort order to move the Compilation selection to be at the top of the list and if checked that it would then place it in the under the "Compilations" section, since this is how their Schema works but only if one or more of the "Song Artists" are different. By moving the checkbox to the top, it would no longer depend on the different artist being a criteria for this to happen to move it to the proper section. This would be much simpler then those of us that have many albums that fit into this criteria without us having to go in and manually make the change. This change in how it sorts internally within the program will have absolutely no effect on how the information they obtain from any service that provides Song/Album information as the Compilation field is not something that these services change. It also does not effect Album Art as this field also plays no part it that either.