It sounds like you're using auto cron instead of an actual cron job.
http://osticket.com/wiki/POP3/IMAP_Settings(http://osticket.com/wiki/POP3/IMAP_Settings)
If you have a real cron job set up, verify that it's working/running without error and that enable auto-cron is unchecked (it won't hurt to have it checked, but may make it more difficult to troubleshoot problems with any running cron jobs).