Some offline songs stop working

[konsum2]

I have a really annoying problem with my offline playlist. From the beginning i downloaded a playlist with about 250 songs. And it worked fine for a quite long time. But now I discovered that songs can't be played anymore in offline mode. The green sign that tells you if the song is downloaded has turn into grey. When i click on the song - still in offline mode, spotify says that track isn't availible. 

But if I change to online mode, the song works when I'm streaming. 

Why in the name of *** does the songs stop working!? 

I've still have days left before I have to go online again..