Squid not caching static content

By looking to Squid3 logs, I've found that Squid it's not caching static resources, for example:

1379041607.923    611 127.0.0.1 TCP_MISS/304 356 GET http://www.deckle.co.uk/squid-users-guide/css/site.css - DIRECT/95.172.21.186 -

The request for that css returns a TCP_MISS (even after reloading), and the same is happening for images, javascripts, htmls, etc. In fact, almost all requests got TCP_MISS with the exception of a very few.

If we look at the headers that deckle.co.uk, we see that it should be caching it:

$ http http://www.deckle.co.uk/squid-users-guide/css/site.css 
HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=604800
Content-Encoding: gzip
Content-Length: 482
Content-Type: text/css
Date: Fri, 13 Sep 2013 03:07:09 GMT
ETag: "d6007-30e-4d2a2f615c780"
Expires: Fri, 20 Sep 2013 03:07:09 GMT
Last-Modified: Sun, 06 Jan 2013 18:34:22 GMT
Server: Apache/2.2.22 (Ubuntu)
Vary: Accept-Encoding

My config file contains:

acl manager proto cache_object
acl localhost src 127.0.0.1/32 ::1
acl to_localhost dst 127.0.0.0/8 0.0.0.0/32 ::1    
acl SSL_ports port 443
acl Safe_ports port 80      # http
acl Safe_ports port 21      # ftp
acl Safe_ports port 443     # https
acl Safe_ports port 70      # gopher
acl Safe_ports port 210     # wais
acl Safe_ports port 1025-65535  # unregistered ports
acl Safe_ports port 280     # http-mgmt
acl Safe_ports port 488     # gss-http
acl Safe_ports port 591     # filemaker
acl Safe_ports port 777     # multiling http
acl CONNECT method CONNECT    
http_access allow manager localhost
http_access deny manager    
http_access deny !Safe_ports    
http_access deny CONNECT !SSL_ports
http_access allow localhost
http_access deny all
http_port 3128     
refresh_pattern ^ftp:       1440    20% 10080
refresh_pattern ^gopher:    1440    0%  1440
refresh_pattern -i (/cgi-bin/|\?) 0 0%  0
refresh_pattern .       0   20% 4320
http_access allow localhost
http_access allow all
cache_dir ufs /var/cache/squid3 10000 16 256

What am I missing? Why am I getting so much TCP_MISS and squid is not even caching static resources?