They were to confirm the date and time for an important planning session. Six responded, and five did not. When I called the five people, they gave me a variety of reasons for not responding. Three were too busy, one did not read my message, and one never received or lost my message. This means making an extra effort, but the results have always been much better for me.
Most people intend to respond, but it gets lost in the multitude of daily tasks.
In my example, I used the telephone for follow-up rather than sending yet another email. It provided an effective communication and it helped me plan a better meeting. I don't have that much time. True, no one has the time to follow-up every message. But we must make time to follow-up the occasional important one.