When THQ's Metro 2033 was patched to support the "Ranger Pack" DLC something went wrong with the game's Achievements. The problem was first acknowledged on August 3 and noted as potentially an issue on Microsoft's end as of August 5. A couple weeks back the publisher told Joystiq it was still "investigating."

THQ has sent us over an updated statement on the issue:
"We understand some people are still experiencing issues with achievements unlocking in the Metro 2033 Ranger Pack DLC. We are currently working with Microsoft to ascertain whether this is due to a code related issue with the DLC itself, or an error in the Marketplace process whereby the DLC was added to Xbox LIVE Marketplace before the new achievement data was added - this exact issue has happened to other games in the past, but we need to be sure that this is what's happening here. We realize this is frustrating for those of you who have been waiting to enjoy the Ranger Pack DLC, and we ask for your patience as we try to pinpoint the exact issue and solution." In the meantime, it's probably best not to download the expansion pack until the problem is resolved. It appears the normal game still works fine. We'll be sure to update once a fix for the problem goes live. Now, remind us, what's the point of Microsoft certification again?