Xbox live settings will need to be set up in the following way to allow TrueAchievements to scan your gamertag.
1. Log into https://live.xbox.com
2. Click on My Account in the top toolbar.
3. Scroll down to Settings and Preferences on the right hand side. Click on Privacy and Online Settings.
4. Scroll down to Privacy. You need to make sure that Profile Sharing and Game History is set to Everyone otherwise you will not be scanned.
These simple steps will allow TrueAchievements to scan your profile and update your achievements.