There are several reasons why this can happen. It can be a simple matter of the landlord being unavailable and not reply within 48 hours, or it can also be because the place is not available after all and the landlord forgot to update its availability. This is the reason why Uniplaces recommend sending more than one booking request to similar places. Keep in mind that if you send more than one booking request, the first one to get accepted is your confirmed booking and the one you will automatically be charged with. All the other pending booking requests will be instantly declined and you will not be charged for them.