Are you in a NTSC 24p or 23.976 project, or are you in a 1080p/23.976 or 24p project. If in NTSC, the TC24 column has to have values in it for all the clips or it will come up empty as you are seeing. In 1080p projects, the START timecode is used.
And, are the missing timnecodes from imported clips or captured clips?
Michael
anything 24fps