A certain number of ressies are held back for onboard booking, and a number are held for concierge guests. If those aren't used in the first day by the concierge, they are released to the general population.
Go to the area for Palo ressies (see the info sheet you will get when you check in for the location) as soon as you board. If you can't get the ressie, get on the waiting list. There is a good chance you will get a call later.