I get that all the time. "Did you get my panicked email that I sent you at 4 AM?!" "Yes. I replied at 7 AM." "Oh, you did?"Sigh. If it was an emergency, wouldn't they be checking their email pretty frequently?
I send out a class email every Sunday announcing any changes and/or reminders about upcoming activities that week. When I ask the classes at our next meeting if they got the emails, half have not checked and the other half is clueless as to what it meant. This means that about half of the class has not come prepared. I tell them to watch for emails from me every week. *sigh*
ALWAYS the case. Always! Yet if someone posts something on their Facebook page, they will know that within 0.02 seconds and will have responded.
Basically, if you don't answer while they're still logged onto email (or whatever the equivalent is on a smartphone -- don't those things let you know when you have mail?), you didn't answer. They're very used to synchronous communication (text or, to a lesser extent, voice); asynchronous, not so much. And they don't want to set alerts for old-fashioned forms of communication like email, since only old farts communicate that way. I think it's the equivalent of not checking snail mail because you know it will be only bills and catalogs (I'm sometimes guilty of that one).
I have no doubt that when it becomes possible to implant computer chips that can connect to the Internet directly into people's brains, they will be a huge hit. When this happens, I will bop around gleefully with a TV remote control, zapping people with microwaves, and giggling maliciously in the manner of a proper mad scientist as they writhe in agony. MUAA-HAA-HAAAAA!!!
I have actually made "looking for my reply" a syllabus clause. I will not allow them to use one-way communications to railroad me.