In my case the emails were picked up, but the thread was not associated with the ticket. If the emails are not being picked up at all, which is a much more common problem, then it could be a cron job is not created, is incorrect, or is firing with errors.
If you are having the same problem I was I corrected it by increasing the "id" increment in the [thread entry] table to be higher than the top "thread_entry_id" in the [thread entry emai] table. In PHPmyAdmin the utility to increase the id increment is here under the tables options. I would recommend backing up your database prior to making any changes to it.
