Trying to split a query into sections of six

I think your approach is correct but you’re running into the “posts_per_page” problem.

In your query, use ‘posts_per_page=-1’ to return all posts (assuming that’s what you want). I’m guessing your website is currently set to show 12 posts per page in the settings->reading area.

http://codex.wordpress.org/Class_Reference/WP_Query#Pagination_Parameters

tech