This happened on my iPhone 4... when I noticed that one of my albums (Album A) only had 2 out of the 16 songs on my iPhone. All songs were checked marked in iTunes on my computer. I looked more closely by going into the "Info menu" on the missing songs and realized that the "Part of a compilation" box was checked on the songs on the "Info tab". I unchecked the box on each song because (Album A) is NOT a compilation of songs...
I then re-synced my phone and the songs showed back up... PROBLEM 1 SOLVED...
However, now an other album (Album B) that I had downloaded displayed the wrong artwork... when previously it was correct. SO PROBLEM 2... or THE ARTWORK PROBLEM...
I read many forums and tried everything and nothing worked... the artwork was correct in iTunes, but wrong on my iPhone.
Then I tried this...
The album that was displaying the wrong artwork (Album B) was a "best of" album... so on that album I actually checked the "Compilation" box for each song by going to the "Info menu" and the "Info tab" and re-synced... then voila!!!
The correct artwork was back!!!!!
Hope this works for you!!!