Not giving me achievements I have earned

Halo MCC is known for having slow to unlock achievements or slightly buggy achievements. My advice would be to try hard resetting your box first, and then loading into one of the missions, beating it again or just quitting out and seeing if it unlocks
then.

You might have to beat each game again or maybe just the last chapters. Achievements sometimes can be delayed by a day or two. I think I had one that unlocked over a week later. But some achievements have to be retried before they will unlock. And
sometimes in rare cases you may even have to replay an achievement multiple times. Or worse case scenario you might have to create a new save file.

Try doing a hard reset without erasing the games

Have a look at this article, it might help
https://answers.microsoft.com/en-us/xbox/forum/xba_gapp/achievement-unlocking-issues-check-here/8a46e4e2-6891-480a-873b-afb962a99642

reset the game and console