previous_post_link is blank

I’m trying to figure out why previous_post_link(); returns blank on all posts after a certain post. I’ve searched the forum, but can’t seem to find any answers for why this might be happening. next_post_link(); works fine.

It works here:
http://www.fangraphs.com/blogs/index.php/cueto-mania/
but does not work in the very next post here:
http://www.fangraphs.com/blogs/index.php/phillies-win-on-a-walk/

I’ve tried this with by my theme and the standard wordpress theme and it’s broken in both places. Furthermore, it was broken in wordpress 2.1.2, and it’s still not working when I upgraded to 2.5.

Any help would be greatly appreciated!

Thanks!
-David