I finally noticed the pattern of which tracks didn't have the running time; any albums where I had the Disc 1 of 1 fields entered the track would show on my iPod as 1-01...., 1-02....... but with no running time next to it, so I removed that tag from each track and now the running time shows on all tracks. I wanted to be able to have it for all the tracks but up until now couldn't work out until now what was causing it to only appear with some of them.