I define a compilation as an album by multiple artists, for example "Alligator Stomp Vol. 1" (a great album BTW.)
iTunes my think differently. For example the Led Zeppelin at the BBC was filed in my hard drive in the compilations folder instead of the Led Zeppelin folder. It can also take a multi disc box set and file some of the discs correctly, and file the rest under compilations.
Go to iTunes>Music>Compilations on the drive where your music files are kept. Open the folder and see what is in there. I found over 100 CDs out of 1300+ filed incorrectly.
To fix it, pull up the album in iTunes, and highlight the entire album with the blue box. Go to File>Get Info>Options and make sure Compilations says "No."
This still may not correct the issue. In this case, click on the album so you can see all the tracks. Highlight the first track. then go to File>Get Info and make sure the "Part of a Compilation" box is unchecked. Then hit next and check all the tracks on the album. Some may be checked, and some not, even within the same album.