TL;DR: Completing the single player game does not earn you the Unwavering achievement by itself, as you only have to complete half of the required missions in order to complete the game.
I believe you are referring to the Unwavering achievement, for either completing all multiplayer missions with a gold rating, or completing all single player missions on the Insanity level of difficulty.
To complete the main story of the game, and in turn earn the achievement for completing the game on Insanity, you only need to complete the core story-related missions. However, there are several more missions that count towards the Unwavering achievement.
I checked out the True Achievements listing for the achievement, since it has been so long since I played the game, and found its primary guide. It has a significant amount of upvotes, and very few downvotes, so I expect it to be very reliable.
It lists there being 13 story-related missions. However, there are 27 missions in total that count towards this objective. Counted missions are listed as "missions where you land on a planet/space station, do something and leave. The random fetch quests for Citadel people do not count."
I would expect the achievement is not unlocking because you simply have not earned it yet. Completing the single player constitutes less then half of what the achievement requires.