People of a certain age will tell you that when they were growing up, most civies replied to any message, even if it was with a polite 'no'.
Now, not replying often means 'no'.
Or it could mean 'busy', 'too busy', 'disorganised', or a 'maybe' if and when they reply.
Once a meeting has happened, I often get a reply.
I guess they get a lot of time wasters. Not nice to be left hanging though. That said, it's just the way it is with some SP's.