I find the same thing with chats, it is pretty much impossible to find a time that works for everyone. Even here at BellaOnline when we have an "editor's chat" that a ton of editors really want to attend, out of 400 editors we might get maybe 15 who can make that exact time and day. So it is just very hard.
An alternative is to set up a forum thread about a specific topic and then make one day - say Monday April 12th - a primary day to post in the thread. So on Monday, when people can, they swing by, see the discussion, add their notes, and feel like they're a part of it. But that way even people who are busy on Monday and who come by on Tuesday can see the discussion and add in their thoughts.