So, me and a friend have, slowly, been trying to get two of the hardest achievements out there, Auf Wiedersehen Petrovic and Wanted. Last night we both managed to fill the requirements of the former, but no achievements popped for either of us. 
 
Has anyone else had this problem? :(

Silly question but are you playing ranked?

Edit:

Auf Wiedersehen Petrovic - 30G
Win all ranked multiplayer variations, all races and "Cops 'n Crooks", as both sides.

http://www.gtaforums.com/index.php?showtopic=341441##Q4

Including races and being on both sides on certain game modes?

http://www.gtaforums.com/index.php?showtopic=504444

This achievement/trophy has been known to be a bit glitchy at times. You might need to redo some or all of the modes.