This happened to me in beta (I think I bugged it but not sure) and again yesterday. Was wondering if it was a bug and if it has happened to anyone else.
So, the first time through it I rescue Marta and it prompts you to beam out and sends you back to Sector space. Once there I see a prompt in the system access pop up that says to continue Roadblock. I also notice that the mission isn't completed and there is not prompt to hail Starfleet. So, I click continue and it starts me in the instance again. After completing it a second time it works and can hail Starfleet to finish the mission.
I've noted that on some occasions, you can turn in the mission before you LEAVE the mission. I've developed a habit of doing this as often as possible. I first had an issue with it way back when the first of the Dominion FEs was released. For some weird reason it'd reset the mission status AFTER the mission was ready to turn in when you left the mission. So I'd just turn it in as soon as it was ready.