My suggestion as someone who goes a LOT.
Cancel your current reservation if you're not going on that day - this will make it easier to rebook.
Check the reservation calendar hourly (especially in the few days leading up). In addition to that, even if it says sold out, as you get closer to your date attempt to book your reservation even if it says sold out on the site - with only 1 reservation, your likelihood of grabbing someone's cancellation is much higher.
If all that fails, sometimes things open up day of after opening. Timing depends on the day.