Yeah, that's odd. It should check every hour. Note that, due to how WP scheduling works, it will only run if someone views a page on your WP site (so in other words, it won't run every hour on the hour, it will run if WordPress gets loaded AND it has been an hour or longer since the last check). Anyhow, if you load up the main blog page and then reload the Post By Email admin page, does it then show that the check happened?
If not, can you install the Cron View plugin and see if the event is getting scheduled? http://wordpress.org/plugins/cron-view/ That just adds a menu option in the admin dashboard that lets you view scheduled events.