I find this very strange, why on earth would a label restrict information about an album they have?
So I asked Allmusic that question and got this response from Zac Johnson, Senior Product Manager: "If I felt as though I was comfortable going into the intricacies of the deals that are in place between a specific worldwide distributor and our data provider, I would detail it further, but the truth is that it's an area that I don't want to poke further."
Example album: "The Definition" by Jon Bellion. http://www.allmusic.com/artist/jon-bellion-mn0003006966/discography (click the album link to see the error message)
To see the response from Allmusic, go here: https://getsatisfaction.com/allmedianetwork/topics/some-albums-are-unreachable
Can HN shed some light on what's going on?