Can I offer at least one suggestion for this? In an age where instant communication wasn't possible, at least not quite so easily, it doesn't seem at all natural to worry when someone isn't replying instantly. If you have to send a letter, wait for it to go through the post (twice) and in between allow your correspondent to reply, then there's no point in being obsessed about whether or not they'll reply instantly, because it's literally impossible. I suspect, though, that if you allowed for the wait then after, say, a week, you might start scratching your head.
Similarly with phone calls: they are around, or aren't, and if they aren't able to answer your call because they are out then, again, what reason is there to be stressed if they can't answer? I think sometimes you weren't even aware if you'd missed a call, there was no notification.
Contrast that with today. Everyone* carries a phone with them everywhere** they go, and instantly*** can be in touch with anyone**** from all over the world no matter what they are doing. Suddenly, instant communication becomes possible. What's more, you can even be aware quite often of whether or not the other person has seen your message, your attempt to communicate or not. If you call them, the phone will at least let them know that they missed a call. If you send a message, then some indication that they have read it will show up on your phone in reply. It's actually rather intrusive to both sides, but the point then is that if you have sent someone a message, and you know they've seen it, and you know they are in a position to reply, and if you know that they also haven't, then not doing so can become rather significant. You might naturally get a little more worried about why someone hasn't replied in such a situation.
That's not quite saying that it's reasonable, but I think you have to make allowances for the fact that the interaction itself is almost tailored to be far more instant than maybe you yourself aren't used to, NJ, before criticising people in this position.
* Exaggeration.
** Also an exaggeration.
*** Another exaggeration.
**** Exaggeration again.