Is your sampler or compilation album (or album with lots of artists) showing up as multiple separate albums in iTunes?
It was happening to me and I was going nuts - why was the new sampler from Paste showing up as 20 separate albums instead of one sampler?
I had check the "Album is a compilation of songs by various artists" check box. That has gotten me in the past, so I know to check for it now.
Then I figured out that the album artist field was filled in. So the compilation checkbox was checked but because the "album artist" field had a different artist iTunes thought it was a different album. Keep the "artist" field, that's ok - just delete the "album artist" field (see the image). Once I deleted the "album artist" field from each song the album was grouped properly as one album.