Slots can be locked for 10 or 15 minutes once someone else selected. Even if you don't see it 6AM EST (HI midnight I think), you might still get a chance 10-ish miniutes later.
Also, unless it's peak peak season, you can see some spots opening up a couple days before as people cancel. I have grabbed such leftover slots in previous stays last-minute. You might need to adapt to the time though