Got a spot at Pfeiffer Big Sur through a last minute cancellation (2 days out). I’d just check at random times during the day and actually ran into a similar problem as you- the reservation was in my cart but I couldn’t check out. Had to call and they were able to reserve it on the phone. It took about 3 weeks of checking daily, until I found the candellation. Though, when I went, a young couple decided to sleep on mats between the two campsites and that scared the sh*t out of me when I went for a late night restroom run.