Slack messages should be considered asynchronous communication, and you should not expect an instantaneous response; you have no idea what the other person is doing.

just ask the question directly, and the person will respond asynchronously. If you truly need to have a synchronous communication, then start by asking for that explicitly, while mentioning the subject. e.g., "I'm having trouble understanding issue #x, can we talk about it quickly?"

Bias towards asynchronous communication Take initiative to operate asynchronously whenever possible