From reports, when you hit book, it will double check for availability again and if in that split second, the room is confirmed from someone else, you get that message because the date of check in you wanted isn’t there anymore...so technically, you are outside the window because the system sees the first date the next day...ie: it’s reading your request as check in November 9th.
I have always keyed up like you and have had success, Just did yesterday with a SV room at RIV, which by the time I confirmed it and went back, to see, they were gone in less than the minute it took me to get mine booked,
I truly think it is luck of the draw for those hard to get rooms!