From their help page: ... (If a submission doesn’t receive enough Diggs within a certain time period, it eventually falls out of the “Upcoming†section.) ... Does it mean it can never hit the front page again? And how long is that "certain time period" ?
Yup. That's what it means. I think the time period is 48 hours, but don't quote me on that. I actually tried to research it this weekend, but found conflicting information.