You have to ask them, Sarah. If you don't get a response within a few hours, message again, asking if they got it, and if so, you need them to reply by XX time, or you'll have to decline.
You can also tell them in the first message that the system is sometimes glitchy, so to please respond ASAP so you know they received it.
Also, some guests neglect to turn on their notifications, so you can ask them to make sure to have them turned on when you send the first message.
And while we tend to assume guests are just ignoring our messages, sometimes they simply are somewhere they have no internet or cell signal for awhile. Or they live in a different time zone and are asleep.
The thing about Inquiries, is that even when you message back someone who sent one, you may never hear back from them again. Some guests send out feelers to several places at once. Or change their plans. Would be nice if they had the manners to at least thank you for replying, but often they don't. So it's good that inquiries only have to be messaged back, not declined.
@Sarah1556