So sometimes there's a problem with achievement syncing to SocialClub - you'll get the Steam's achievement but it does not register in SocialClub. Has happened to me with "Auto Fanatic" and "Bulletproof Windshield".

This is due to sync-triggering by L.A. Noire's engine. Sometimes you have to go to some checkpoint location, end current event or something like that to trigger it. Quickest way is to go to game's menu after Steam's achievement is shown, then change something (turn auto-aim on for example), exit menu and game will sync.

edit: just checked it out again. Same thing was with "Roscoe And Friends", used the flamethrower and Steam ach. appeared, nothing on SocialClub's side because no sync event was triggered - so went to display options, changed it to Black&White and voila, synced.

Works for me.