If you see unexpected groups of songs when you browse an album - https://support.apple.com/HT204538 - "Songs from the same album might not be grouped together as you expect when you view them in the Albums view in iTunes."
For most people this solution seems to be the one that works: Aug. 2008 post by turingtest2 about temporarily changing labels - https://discussions.apple.com/message/7904806#7904806 or http://www.samsoft.org.uk/iTunes/merge.asp - Add a letter at the end of the artist name, close the Get Info window, then open it again and remove the letter. This often works but I have had cases where tracks grouped when a letter was added but ungrouped when it was removed again.
The next sections provide additional solutions:
Often a solution: Select all the tracks on the album, File > get info (or right-click > get info > details), and either give them all a single "album artist", or check the "compilation" flag (as in https://discussions.apple.com/message/17670085#17670085).
Steve MacGuire a.k.a. turingtest2 - iTunes & iPod Hints & Tips - Grouping Tracks Into Albums - http://www.samsoft.org.uk/iTunes/grouping.asp (older post on Apple Discussions http://discussions.apple.com/message/9910895#9910895)
If these are from multiple-CD sets you may also need to enter the appropriate information in the disc number fields. If they are not a multiple CD set you still need to make sure the disc number fields are set correctly or all empty.
Another explanation by hhgttg27 August 2015 - https://discussions.apple.com/message/27784417#27784417
If they are mp3 files try changing the ID3 tag version which may cause iTunes to re-write the tags and clear up inconsistencies. I use iTunes 7.5 (yes, from 2007) so I can't tell you where this is in later versions.
I had one case of really stubborn grouping. The tracks would re-group in ways that didn't make sense when I changed metadata. Without going into detail, I deleted the tracks completely from iTunes, then re-added the files and they all grouped correctly. Doing this will result in some metadata loss but if you have no choice...