Hi, sorry I can see this has been marked as resolved, but I thought I might add something here. If you don’t see this Paul, I will post another entry separately tomorrow.
I had this problem myself, and whilst I can’t tell you what is wrong exactly I can tell you what I think it is associated with Paul, which might help you understand why it might be happening (I fully appreciate it might be a theme thing though and nothing to do with achievements).
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).
So if I have 18 achievements and Blog is set to show 17 posts, achievements shows 15, then pagination works and shows final 3.
If Blog is set to show 19 posts, achievements again shows first 15 but then pagination does not work – it can’t find the second page because it assumes as there are 18 achievements it has shown them all I think.
So I guess the question is does achievements show only 15 posts per page regardless of blog settings – I think this is why pagination MIGHT be breaking.
Hope that helps.