Magento

I have setup a set of rules to apply a sales price on a range of products. The problem is that these rules cease to apply without reason, after a while and I have to reapply them again and again in order for them to work.
The rules are set to expire after a long time, yet they seem to stop working at random.
Is this a bug? Is there something that prevents sales rules from applying?
Please help!

Have you set up a cron job to process the rules regularly?
http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/how_to_setup_a_cron_job

Regards