I had a recent experience where I saw C&P availability for a given night appear and disappear a couple of times within a week or so. At first it was consistently available; I was waiting for points to post. Then it disappeared for days, only to reappear for less than 12 hours, then disappear again right after my points posted. I kept checking, though, and the next day it came back. I don't know if this was just a fluke (some individual making and canceling the last available C&P reservation) or if this often happens. But at least in this case it was well worth checking back.