As a junior i often wrote "Hi, do you have a moment? (ok if not)" in slack (or lync at the time) to senior people when I had questions. I then didn't expect a reply unless the senior person was not busy or bored. If no reply within 5 mins I would ask the next person. I did this when I had a question that perhaps 20 people who I knew could answer, though I had no way of telling who (if anyone) was free to chat somethi…
The optimal approach is to ask your question in a public channel . Then all 20 people get to see it at once, they can see if someone else already responded and other people on the team who may have been wondering about something similar also get to see the question and the answer(s).
Asking someone directly almost always results in a quick response, even if it is a "I don't know" answer. And an "I don't know" from a senior colleague can mean a lot, for example that the problem is much harder than you initially thought.