So I have read a lot of comments over waypoint concerning achievements not unlocking. The two acknowledged bugged short term and long term commitment achievements have been fixed and are now unlocking for some.
However for the rest of you guys and gals experiencing issues I will post a guide. First of all have a look at your Halo Wars 2 service record. If it is saying you have not completed a mission or collected an item go back to your game and give it another try.
You must not complete the campaign offline. This will void the achievement. If you have done so, you will need to redo the mission.
Also some multiplayer achievements such as ‘The roof is on fire’ won’t work in blitz. Until certain achievements are patched in the near future other ways of progressing achievements will open up.