Some songs will not play???

I recently imported songs from albums onto my iTunes library but when I transfer them across to the Ultramixer playlist, they register but it says that the length of the song is 0:00 and will not play where as the rest of the songs which are on the iTunes library from different means (youtube conversion) will play on Ultramixer.
Is there any way to rectify this to get those imported songs to work as this is causing quite an issue?? Thanks … ic-66.html