Just got back from Thailand. Same problem here.

Go to a booking site (low rates) which wants you paid in advance and it came back saying that the room (using internet rates) is not available and want me to upgrade. Of course, I said no and asked to clear the CC blocking charge which is not charged till a room is completely confirmed.

I went to another site (slightly higher rates) and got instant confirmation. You only have to pay at checkout time.

Yes, there are many booking sites, some are less “real” as in real-time updates.