Trying to get a campsite at a California State Park can really be an exercise in patience, they seem to go out of their way to make it hard to get into them without a reservation. I hate having reservation, takes all the spontaneity out of travel. Some were worse than others as I remember.
We have stopped at campgrounds and been denied a site because we had no reservations.
We simply went back to the TV and called in for a reservation. This has worked for us more than once!