I used to deal with anxiety about this a lot, so I know what you mean. It still hurts a bit when, like you said, you know they saw your message but then they post something to Instagram or whatever. But I think the fundamental cause of this anxiety is that we're ascribing our own priorities and ways of doing things to other people who are fundamentally different in their personalities and priorities. I think it's rooted even deeper with the idea of being emotionally dependent on others.

I'm generally one of the "text back ASAP" types, so it always feels a bit shitty when I text someone, they reply 4 hours later, I reply in a few minutes, they reply 4 more hours later, etc. But ultimately it's just something that you can't change, and while it's often comforting to look for a reasonable explanation (they were working, in class, whatever), working at the deeper insecurities is more productive. That is, in the end, why should it even matter if they text back fast? I am my own source of happiness, I shouldn't need someone to text me back more quickly to ease my anxiety.

But getting down to it, there are the more literal practical reasons. If I'm out with friends or doing something that requires a fair amount of attention from me, I generally won't respond to a text unless it's time-sensitive just because I think it's rude or distracting to attend to something that can wait until later. So that's often probably the case.

Unfortunately I don't think there's any easy fix to getting over this type of anxiety other than trying to keep busy and interested in what you're doing in the moment, as well as recognizing that everyone shares different priorities. Good luck! :)