MySQL slow query logs not working

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http://serverfault.com – I need some help diagnosing a fault I have a development server and a production server. They are (in the large part) identical in setup. Both are Ubuntu Server 10.04 LTS installs. I turned on the MySQL slow query logs on both servers, and it works on the development server, but not the production server. On development, the slow query log fills up fairly quicly, but on production, the log file remains empty. And idea how I can diagnose why it's not working on production? Are there any error logs that might contain the information? There's nothing obviously relevant in /var/log/mysql/error (HowTos)