Unlike reservations for dining in the parks, which have a fee for same day cancellations or no shows, there is no such fee for dining at Aulani. The reservations may be filled, but it costs guests nothing to cancel at the last minute or not show up (edit) so they tend to hold them "just in case" until their plans firm up.
We got into a Menehune Mischief dinner show by periodically checking with dining, and they ultimately were able to accommodate us when we called in the early afternoon on the day of.