I've just completed Act 1 – and looking at the achievements, it seems there are several which have not awarded because the requisites have been marked as not completed.
For example: The Last of the Horadrim Achievement which says complete all quests in Act 1, has few quests marked as incomplete even though I I'm pretty sure I've completed them.
Why is this so?
Best Answer
The servers, overburdened as they were, had a hard time synchronizing all achievements the first few days after release.
Official Blizzard response
I had the same issue, and actually had to redo some bosses in Act I to get the "kill boss X" achievement.