Find & recover missing achievements & perfect games.
Update all trackers of your choice after each gaming session to make sure no stats get lost. Unfortunately Steam is prone to not reporting stats of games that are not permanently linked to your account's library.
NOTE: Do not remove all unlocked achievements to raise your AGC (Average Game
Completion) as this will in fact have the opposite effect.
Games that had unlocked achievements before will be calculated into AGC with 0%.
Some of your stats might be "hidden" within the Steam APIs. There are various methods to extract those but none of them is bullet-proof; so some stats might just be missing forever.
However, you may find links to various recovery methods in the navigation bar above (Steam Profile, Steam Store).
Zero Playtime & Offline Played Games
If you play games offline on a regular basis (not recommended in general as timestamps might
lost or scrambled) check for these first. Usually trackers do not fetch data for games that
playtime recorded to reduce unnecessary load on their API crawlers.
Most trackers allow to update a single game. On completionist.me go to your game's detail page and hit the " Update" button at the top right (which is available as long as no other update job is queued for you).
Alphas & Betas
If you happened to have participated in closed alphas and/or betas, that are not accessible anymore through Steam, those achievements are gone for good.