Welcome to the boards, Dan Williams.
When you landed on NGC-1027, did you still have cargo in your hold? If so, then you haven't retrieved all the cargo, or else you landed on the wrong planet. If you don't have any cargo when you landed, then somehow you failed the mission. Is the mission still active for you? If not, you failed it somehow, either by taking too long or accidentally shooting one of the ships you are to disable and board, and you can pick it up again.
Do you have to disable three or four ships? If four, do you have enough cargo space for all four ships' cargo? I think each ship you board has 5T of weapons, so if you only have 15T of cargo space and have to board four ships, you won't be able to finish the mission. It also seemed to me when I had exactly 20T of cargo space I still wasn't able to finish, I had to have 25T cargo space.
If you let a transport get away, you should be able to jump out of the system, then jump back in and the ship(s) should be there.
Good luck, let us know what happens.