Multi-Part Job Completion

We're using mult-part jobs for our production.

Most of the time these jobs will consist of one operation only, and when these are released and a user gets to the stage of End Labor Activity then they can see all the job parts against which they can update quantities produced. Thats fine and what I would expect.

Just occasionally however some of these jobs will have more than one operation. In these cases when it comes to End Labor Activity no job parts appear in the panel, its as though its seeing it a normal "single line" job.

Can someone explain why this is happening, where I'm going wrong ?

Many thanks