This thread might come in handy. I'll start this one off.

The Hyatt Valencia and Santa Clarita does NOT participate in web checkin.

I originally booked an AAA rate which was not eligible for web checkin anyway, but noting the WEBIN bonus, I changed the AAA rate to a prepaid hyatt.com rate (a few $ more expensive). Only now, I have been informed web checkin for this hotel is not possible. For whatever reason, I don't know.