Please help as I am frustrated!  I have some local files that I have imported into Spotify.  I sync them to my iPhones and they stay for awhile (several days to a couple weeks) and then randomly the iPhones will say the file is unavailable and has to be synced from my computer.  That's annoying enough.  But then when I go to try to sync again they don't sync.  I have to remove them from Spotify and start over completely adding them into playlists.  This is unacceptable to me.  I love Spotify but this issue is enough to make me cancel and try something different.  Any idea why this is happening?  It happens on everything from Taylor Swift to The Beatles.  The strange thing is though it doesn't dump all local files, just some (like Taylor Swift's newest album but not all Taylor Swift files).  Any help is appreciated!

I'm on Windows 8.1 and iOS 8.2.