@Sage28 When guests don't respond to your messages, it could just be a matter of them not feeling that a response is necessary. Sending check-in info may, in their mind, not require a response, not even the courtesy of a thank you.
So it's best to ask for a response, rather than just assume they got and read what you sent- "Please let me know ASAP that you've received these check-in instructions so I know you won't have an issue with retreiving the keys at the time the key cafe will be open, and checking in."
Making things about you being concerned about the guests' comfort and convenience, works better than it being about the host's expectations.
If you do that and still don't get a response, try sending them a text message to their phone number asking them to respond to the Airbnb message you sent, and if they are still not answering, ask Airbnb to try to contact them.
Non-responsive guests are a red flag and I wouldn't just ignore it and assume all will go smoothly.