.htaccess/expires headers issue

I’m trying to optimize my site, so I created the following .htaccess file in my web root (https://expat.cl):

<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault A300
ExpiresByType application/x-javascript A3600
ExpiresByType text/css A3600
ExpiresByType image/gif A3600
ExpiresByType image/png A3600
ExpiresByType image/jpeg A3600
ExpiresByType text/plain A300
ExpiresByType application/x-shockwave-flash A3600
ExpiresByType video/x-flv A3600
ExpiresByType application/pdf A3600
ExpiresByType text/html A300
</IfModule>

That, predictably, broke my permalinks, but I regenerated them and all was well. The file now looks like this:

<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault A300
ExpiresByType application/x-javascript A3600
ExpiresByType text/css A3600
ExpiresByType image/gif A3600
ExpiresByType image/png A3600
ExpiresByType image/jpeg A3600
ExpiresByType text/plain A300
ExpiresByType application/x-shockwave-flash A3600
ExpiresByType video/x-flv A3600
ExpiresByType application/pdf A3600
ExpiresByType text/html A300
</IfModule>
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

YSlow is telling me that I still don’t have my expires headers set (I do have some set via the Autoptimize plugin).

Is there something wrong with the mod_expires settings above? Why is my server ignoring the file?

Thanks