@Stephen720 If the guests fail to contact you with their scheduled arrival time, all you can do is shift from Asking mode to Telling mode. Give them a narrow time window in which you can check them in (for example, the last two hours of your normal window) and inform them of the check-in procedure.
That usually triggers uncommunicative guests to reply. But if not, at least it's ok the record that you made the attempt, so the guests can't make a valid refund claim if they don't turn up.