A lot of people have literally no idea how to react to the news of a bereavement these days, so they do nothing.
This is extremely hurtful to those of us who were brought up to express sympathy either in writing or face to face the first time we saw members of the deceased person's family or circle of friends.
There is not a lot you can do about it.
Whilst I was teaching I told older pupils when the subject came up, which it could well, as I both taught English literature and Religious Instruction, that the only thing you must never do is to do nothing. Any expression of sympathy such as I was sorry to hear your sad news cost nothing to say and may mean the world to the person you say it to.
You are justified in asking those who did not reply to your text whether they actually saw it, next time you meet them.
Regarding more mundane texts that go unanswered, I assume the person I texted had been too busy to answer. Not very polite, by the standards of our generation, but acceptable, as far as I can gather, by those younger than ourselves.
Not a lot of point in mentioning that kind of negligence. If you really do need an answer. phone and ask for one.