In the Avalon Cup I was getting a bit stressed with similar things, but as it turned out, I was sending messages to someone's brother. It was the brother online all the time and not my opponent. Maybe send a messages asking if that is the case for any of your people who aren't replying. It could easily be true.

But yes, overall this is always the problem. A lot of people are enthusiastic as long as they only have to say "I'll play", but they don't actually want to be involved in any of the planning. They literally just want to show up and play.