It could have something to do with your sorting. If not, look at the steps.
1) Make sure all songs have the same EXACT spelling of album title and artist. This includes an extra spaces after, in between, or before in the album title or artist.
2) In one set of duplicates... highlight all tracks, change the album title to something stupid like "Apple"
3) After renamed, highlight all tracks, change the album title back by manually typing it in (it should grab the one that is out there), save. They should be grouped together now.
Step 2/3 happens to me alot of the time when I change the Artist field from having multiple artists to the main artist and adding "(Feat. ....)" in the Name field instead.
In example...
If you buy Kanye's Runaway track, iTunes put the Artist as Kanye West & Pusha T
I update it so Artist states Kanye West and Name as Runaway (Feat. Pusha T)
This step always throws off the album into duplicates, at least for me.