For those of you who play the reboot via steam, if you have had issues with the Achievements unlocking follow these steps. I tried em out myself and they work.
- Browse to your Steam 'userdata' folder - Go into the '960533' folder - Go into the '203160' folder(This is the Tomb Raider save data folder) - Go into the 'remote' folder(This is the Profile and Save games folder for TR) - Backup and delete everything in this folder to reset your profile and save games - Go to your Tomb Raider game in your Steam Library - Right click and select 'Properties' - Click on the 'updates' tab - Make sure that the 'Enable Steam Cloud synchronization for Tomb Raider' box is NOT checked
Start the game and start a new game. Once you fulfill the missing achievement(s) they will be unlocked.
The main issue that causes the achievements to glitch is the Steam Cloud Saving, it has nothing to do with the game itself or anythin CD did. You'll know if you did everything correctly if while your playing you start unlocking concept art etc.