Iphone 4S: Synched local songs become "unsynched" after 1/2 weeks

[Datmix]

Hi there,

I have a problem with Spotify using my Iphone 4S.

I add some (around 20) local files to my playlist on my computer. Then I synchronize the playlist on my Iphone.

First, everything works fine for a couple of days.

Then, after 1 or 2 weeks, the local files simply become "unsynched" on my Iphone, which means that they are still listed but I can't listen to them. The titles are now gray but the green arrow is still there. When i click on those songs it says "song not syncronized".

Furthermore, if I want to make the songs available again, I have to synchronize the whole playlist again - even the non-local files.

On my computer everything works fine.

What am I doing wrong? I have restarted and reinstalled both the desktop-app and the apple-app.

Thanks for helping, hope somebody knows a solution..