if you write "hello" and they answer "hello", they are under some social obligation to answer rather promptly, whereas if you write your question first, it may simply get ignored. So I tend to write a short hello first.
possibly some other social factors at play here, but still, my experience.
That's exactly the problem, though. You're expecting prompt replies in an async communication medium. Ask your question and then they'll get to it when they have time. If they never reply when you do that then there's a larger issue, but that rarely ever happens. People expecting immediate replies in Slack is a big issue that leads to a lot of unneeded stress at work, and decreases everyone's productivity in the long run.
I dont expect an immediate reply, just within reasonable time. I i've made the experience that a question you ask upfront simply gets ignored and then forgotten.
When that happens to me, I'll give it a reasonable amount of time based on the importance of the question (which could be a few hours or a few days) and then gently prompt with "hey, sorry to bug you, wondering if you had a chance to look at my question?". That has never not worked out for me.
If it's such an emergency that it can't wait, then I can call them. But most of the things we think are emergencies aren't, and most things can wait longer than we think.
possibly some other social factors at play here, but still, my experience.