No time data stored, causing issues with itunes?

I imported my entire library of music into songbird and some of it didn't show any time details in the column. I tried reimporting the library with itunes and not all of my music came back. It appeared that the songs that were missing were the ones that didn't show any time. Any ideas on where the issue comes from?
Oh, and the issue of there being no time goes if you play the first few seconds of the song. It realises how long the song is and the data returns.