Steam recently changed the default privacy settings for all users. This may impact tracking. Ensure your profile has the correct settings by following the guide on our forums.
I'm not sure if it's just me or if other people have encountered this, but the achievements earned doesn't always match the actual number of unlocked achievements. For example my Party Hard tracking say I've unlocked 44 achievements, when there are only 26 in the whole game. (Prison Architect...