Thanks Norm,
Actually the problem was a really strange one to resolve.
The files were not corrupted in anyway. But because I had previously played these songs on my iPhone (using the music app, streaming them from the cloud) since the iPod had last been synched with my library, the original files in my iTunes library on my computer must have somehow become modified because they were now not recognised by my iPod classic when it was plugged into my computer/iTunes where the original files are stored.
The way I resolved it was to find the files that were not synching in my iTunes library on my computer, and play a few seconds of each of them IN ITUNES ON THE COMPUTER. By triggering the file from the computer, it then became recognised again by the iPod when plugged in.
This has rather put me off using the Music app or subscribing to the new Music app, and streaming my library on other devices like my phone and tablets.
I don't want my iTunes library manipulated or for certain files to be treated differently, and not be unaccessible should I ever stop subscribing to the monthly service. I like having my music stored in a place that isn't susceptible to all these issues.
Thanks a lot for responding and the kind offer of help.