Posts stopped Archiving by Date

My company recently migrated our blog from Blogger to WordPress. We first imported posts in April, then imported the rest in July and have been since posting from WordPress itself.

Our archive by date works fine for every post from the first import, but does not show any of the other imported posts or any new posts that we make from WordPress. All of our recent monthly archives show a 404 error page.

Here is our URL: http://blog.localwisdom.com/
2009 archive showing only until late April: http://blog.localwisdom.com/2009/
The category archives work perfectly as seen here

Another thing I see working at random are tags. For example, in our technology tag archive, no new posts show (and there are plenty of posts in it after April). But in our design tag archive only a few new ones show when there are many, many new posts between April and now. Then you have our photography tag archive that shows every new post (but the archive was created and used only after the WP conversion).

Weird, right?

Anyone have any ideas that can help me fix this? Thank you in advanced!