All the songs on the album need to have the compilation flag set and the text "Various Artists" added to the Album Artist field.
In the screenshot below I have selected all the songs on the album Absolute Soul and then clicked on Edit/Get Info (from the iTunes top bar menu), which shows the view you see here:

Notice the two fields that I've ringed in orange. Make sure that Album Artist (note, not Album, or Artist, but Album Artist) has the text "Various Artists" in it and that the compilation flag is ticked.
- If the Album Artist field has "mixed" in it, this means that some tracks have different information in that filed to other tracks. All tracks on the album need to have the same information.
- Also, if the compilation flag has a dash (-) in it, this means that some tracks have the flag set, while others do not. Again, all tracks on the album need to have that box ticked.