Certain songs won't play on devices

I have an iphone 5 and an older ipod colour 30gb.
Recently the iphone wouldn't get new songs added to it during syncing with itunes.
I removed all music then readded it. It now has all songs displayed on it, but will only play some.
The ones which don't play have the stop icon next to them in the playlist view and are greyed out.
It just skips to the next song when attempting to play. These songs played previously and are fine in itunes.
They are imported not bought MP3's.
After recently syncing the ipod recently it has the same issue, showing songs in the list but just skipping over them when playing.
I am loath to restore both devices, as it would seem like an itunes issue if it is affecting multiple devices similarly.
Is it possible to rebuild my library or something, or should i restore itunes?
Im a reasonably experienced mac user so have checked what i hope are sensible things, inc d
Ideas?
Thanks!   

In addition I have now:
Reinstalled itunes,
rebuilt my itunes library,
restored my iphone and used a backup on it.
It is still doing it! The songs which won't play or are greyed out etc are different this time again though.

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