Notably, new information might help:
"Setting the WordPress Read Settings to show more posts per page than you have achievements, breaks the achievements pagination (or at least that is what is happening for me)."
This still exists as per description, but haven't been able to understand why. It works fine in bbPress, so I've either broken some code when I copied/pasted something, or missed copying something all together.
Investigate this again: http://wordpress.org/support/topic/achievements-page-2-and-3-doesnt-work
Notably, new information might help: "Setting the WordPress Read Settings to show more posts per page than you have achievements, breaks the achievements pagination (or at least that is what is happening for me)."