I've ran into this problem as well, and it usually does stem from having a typo in one track's artist or album artist (even just capitalization). One album of mine had track 2 as a separate album, but the rest all together. Trying several of the solutions around the web only resulted in accidentally separating track 3 as well.
I had gone through and selected all the songs from this album and made sure that every field in the metadata was identical (except track names and numbers, of course) but no change. In Windows Explorer, you could even see that iTunes had already sorted them into the same folder. To be extra sure, I even used MP3tag to verify that all the necessary fields were identical.
The only thing that I have found to do is delete the album, making sure to select Keep Files (DO NOT send to the Recycle Bin), and then drag the tracks from their original folder and drop them on iTunes. This will recreate the album in the list, and they all come out as one album again. It is certainly an annoyance, and something Apple should have fixed by now, but at least this method has worked for me every time.