Apple has released iTunes 12.4 with some design tweaks to simplify the previously ghastly interface. The most notable change is the return of the sidebar as a prominent default screen element. Apple replaced the media kind buttons with a pop-up menu, called the Media Picker, for choosing which media type to view. The company also eliminated the View Options menu — instead, use View > View Options. Finally, the Back and Forward buttons now work across the entire interface, instead of just in the iTunes Store. For more details, see Kirk McElhearn’s write-up for Macworld, as
well as a list of minor changes on his personal blog. (Free, 233 MB via direct download or via Software Update, 10.8.5+)
Several albums, including compilation "albums," were missing from my modest collection of mostly classical music in iTunes upon completion of the update. Had I not read Kirk's Macworld article, I would continue to assume they had vanished per the recent experience of some iTunes users. Once I had edited the "Library Menu," not only did my big compilation become visible in that category but also did other albums -- probably all of the missing ones. And those are not -- I repeat, NOT -- compilations. They are ALBUMS, some of which were purchased as such in the iTunes Store. It's just that they have a chorus and conductor on some tracks, soloists-conductors on others, soloists-chorus-conductor on others. That, by iTunes' peculiar definition, makes them "compilations," and try as I might I cannot make them "albums" unless I edit all the tracks at the same time, force the "Album Artist" to be the same for all tracks, and removed the faint "Mixed" from the "Artist" category at the top (though this process in one case has deleted important information about performers). This feature represents another bug that needs to be fixed for classical music lovers. Or does iTunes/Apple merely wish we would go away to nurture our respect for "composers" just above "artists"? Sorry for the length of this....
You don't say if the songs' Compilation checkbox is unselected?
The checking or unchecking does not seem to be the determining factor. I have been able to resolve the placement of the albums by various methods: deleting the faintly visible "Mixed" in the Artist category or by removing the download from my iMac and re-downloading (yes, I subscribe to iTunes Match) so that the same album that was in Compilations on my music-in-the-cloud Macbook Air would appear in the same category. That move got both to appear in Albums rather than one in Compilations and the other in Albums.
Now what is interesting is that on my iOS devices, as has been the case for a while, some albums appear in Compilations, but they appear also in Albums. That duplication was not the case, as I indicated above, in OS X in iTunes 12.4.