Money would be the short answer. It's also possible artists that have been on different labels have properly negotiated rights for some albums and not others. Just a few weeks ago, the Pink Floyd station would play cuts from keyboardist Rick Wrights last album. I noticed when I wanted to add a couple of the better tracks for one of my stations that all of the sudden slacker was giving me the "currently negotiating licensing" business. Contract must have expired.......
Personally there are a handful of songs I couldn't find by assorted artists. Some artists run deep and some are lacking. I noticed there were a bunch of Neil Young tunes / albums missing. Not being able to tell if it's a live version of a song can sometimes be a pain too.
I have a soft spot for mid 80's hiphop, was in middle school at this time. I used The Fat Boys as a station and used the other artist Slacker suggested to make a station. I get one song repeated form 2 different best of CD's, and that's it.
I can really speak to any specific recordings, but I do know that in the New Age, Southern Gospel Genre that there are a lot of artist totally missing from the service.
I have posted artist list just using the latest Top 40 Southern Gospel chart toppers and out of that group there is like 7 artist on Slacker, the rest do not exist.
When you post in the forums the PD's don't respond. I've heard the support guys make excuses for them saying they have other jobs. But how hard can it be to get online for 30 minutes a week and respond to questions. I have to call BS on that.
In regards to most other Genre's I've been very happy, but have noted an artist or two missing here and there. I have also found that if I am hearing repeats, that I need to move my settings to auto because when I have it on hits or familiar then I am cutting the library almost in half. I am still very happy with Slacker for music. It isn't perfect by any stretch, but I am sure enjoying it. This morning I've been listening to a Vintage Country Station I made. WOW, I've heard artist and songs I've not heard since I was a little kid. Boy Patsy Cline, she had a voice sent from heaven! Amazing the music by her I've never heard play today on my station.
The last bullet may be a different number than 100 if you have a paid "account". Mine's the free one and so far no matter how prolific the artist, when I edit a station to "Request Songs" the most songs I will see is 100. I've also noticed that a song was missing from that 100 one week, then another week it was there.
Re live versions, the behavior I recall is that in the "Request Songs" list there might not be a designation that it's the live version. Then later when you see the song listed under "Edit Ratings" all of a sudden it's designated as live. I also seem to recall that when it played on my portable, heart symbol and all, it wasn't the live version. Round and round, with the confusion...
P.S. I started a thread about this and similar issues. Please add your own observations there.