If anyone else has had this issue;

You think you've completed the job and go to the PC and call up the Active Contract in the Organizer to sign it off and collect your money, the checklist under "Required Repairs" says, for example, that the Driver's Hatch on a T34/85 is missing. So you buy one and then go back to the tank, set your interface to Assembly Mode but there is no shadow over the Driver's Hatch for you to connect your purchase to.

A solution is to change to Disassembly Mode and remove an item, any item, from the tank then switch back to Assembly Mode the assemble "ghosted" Driver's Hatch will appear over the Driver's Hatch, enabling you to attach the replacement and then fully complete the job in the Organizer and collect your money.

Sorry if this is a known issue and solution, but I'm new to this fantastic game and thought it might be of help to other newbies who may encounter a similar problem and getting unnecessarily frustrated as I did. I hope this helps someone.