FastCGI cache has stopped working

Nginx's FastCGI caching used to work perfectly for me, but recently it stopped working and I can't work out why.

This is how the headers look:
HTTP/1.1 200 OK
Server: nginx/1.2.7
Date: Sun, 03 Mar 2013 12:28:24 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 10727
Connection: keep-alive
X-AspNetMvc-Version: 4.0
X-AspNet-Version: 4.0.30319
Expires: Sun, 3 Mar 2013 13:28:24 GMT
Cache-Control: public, max-age=3600
Last-Modified: Sun, 3 Mar 2013 12:26:20 GMT
X-Cache: MISS

It appears that the Last-Modified, Expires and Cache-Control headers are set correctly, however the cache status is always "MISS" and the cache directory is empty. Any ideas on why this would be happening, and how to debug this?

The site URL is http://dan.cx/, and here is my Nginx configuration for this site: https://github.com/Daniel15/Website/blob/master/nginx.conf

Thanks!