Many users have reported problems accessing songs in their iPod library. There are several
bug in Apple's iTunes match, iCloud, and iPod Library access framework. I filed bugs reports
and am working with Apple to resolve these problems.
The problem typically happens when either the song has not been down loaded to
your device or has become corrupted. Apple does not provide a way for Slow Down Music trainer
to down load the song for you if it is not already on your device.
You can work around this bug by using Apple's built in music player.
start Apple's music player
tap on the "more" on the bottom of the screen
tab on "albums"
select the album containing the song you want to use
Down load the entire album. down loading just the song does not resolve the problem
You should then be able to start slow down music player , press tap,
"iPod library" to add it to your list of favorites
I Personally used iTunes wired sync once just to cause all my music to down load to my device.
See Apple's support page iOS: Syncing with iTunes
You might also try using the settings app to turn "iTunes match" off for a while. Also
turn off "show all music". This is tricky. You need to make these
changes in the setting for "iTunes and App Store" and "Music"Then try starting slow down music, tap select tap
"iPod library" and select the song you are having trouble with again.
When you turn "show all music" off. You will not longer see songs that have not
been down loaded to your device.
If you purchased this song a long time ago there is a chance it is really DRM protected.
See how to remove the DRM.