Why were singles removed from some albums?

[ChunYinZi]

It's a well known fact that some library music albums get taken off the shelves and you can't listen to them except by buying them on CD or saving them from many years ago when they weren't taken off the shelves

But I've noticed that some of the albums that can still be viewed online have some singles that are gone, but the albums are still there, why is that?

For example, DE WOLFE MUSIC 

DWCD 0361 Classically Chilled, 13 and 14 tracks are gone from this album.
 
They are called Pie Jesu, Pie Jesu (Alternative Mix).

I found them on the CD but not on the website.

Why were they removed?

What other libraries have them?

We have some unknown sources of music, could they have come from these disappearing singles?

[lbjames]

I think we need to compile a list of missing tracks....whether it's from de wolfe music website.......or APM or universalproductionmusic etc.

[Craig-UK]

Tracks can be removed for various reasons, these are just a few reasons:

• Copyright reasons
• Contracts could finish with composers so they remove the tracks
• Disputes with composers could make them pull their music
• The library may think that the music is just not worth having available due to thinking they have not aged well
• The library may have a reshuffle and just have not yet found an album to put the track on or may never do so

That is just some of the reasons and there could be many more

[ChunYinZi]

Tracks can be removed for various reasons, these are just a few reasons:

• Copyright reasons
• Contracts could finish with composers so they remove the tracks
• Disputes with composers could make them pull their music
• The library may think that the music is just not worth having available due to thinking they have not aged well
• The library may have a reshuffle and just have not yet found an album to put the track on or may never do so

That is just some of the reasons and there could be many more

Very good answers

After my tests, these dropped singles are not recognized.

Uploading them to youtube doesn't give any indication of copyright.

So I'm going to organize some of them in the near future and see if there are any singles from those albums that have disappeared.

[Retromatic]

Based on ASCAP results it seems like certain companies and artists might have bought the rights to some tracks and albums. For instance, certain tracks might become unique themes for TV shows. But I know nothing about the legality of these things. I've also seen some libraries re-branded under different companies.