Apache not logging user agents

[Gunther Mayer]

Hi there,

I'd like to start using some log stats tools like webalizer or awstats 
but they need the right information in the logs to build stats from. I 
checked my httpd.conf file (it's still very close to the default config) 
and it correctly specifies the NCSA extended/combined log format:

LoadModule log_config_module libexec/apache22/mod_log_config.so
<IfModule log_config_module>
    LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" 
\"%{User-Agent}i\"" combined
    CustomLog /var/log/apache/httpd-access.log combined
</IfModule>

However, most of my logs entries end with %>s %b without any referer or 
user agent info at the end, these only appear very haphazardly (e.g. on 
30 June only 26 out of 1801 entries include referer and user-agent).

Does anybody know what could be wrong?

Gunther