When was the last time you organised an event for friends or family, sent out invitations, and received an immediate 100% response? It’s probably never happened. So you contact the non-responders in other ways, perhaps by a phone call this time. Are they irritated? No, of course not; they’re apologetic, but more importantly, grateful you’ve reminded them that they still haven’t responded.
However, when it comes to marketing, most companies are really scared to follow up non-responders. If your offer is good enough, you shouldn’t be. Indeed, your audience might be grateful you’ve reminded them.