Could be a few things...
1) The track encoding; what is the file format of those tracks, and if it's MP3, what encoder was used to create it?
2) Bad metadata in the track; does the file contain any APE tags?
3) Cross-linked song ID in the database; if you previously imported an identically named song from the same location (eg. you have used the same name for a sample in the past), it could be looking up some old data and thus not knowing how to play that track properly. You could try simply moving or renaming the source folder and re-importing them.