Afolasayo, getting the right product champions to participate can be a challenge. I discuss this in more detail in Chapter 6 of my book Software Requirements, 3rd Edition. If you can identify good candidates but they don't want to spend the time, point out the risks of having inadequate or inaccurate user representation: primarily building the wrong thing and spending unplanned time and money on fixing it afterward. Like I said in this article, you're going to get the customer input eventually. It's just cheaper and less unpleasant to get it earlier instead of later.

When I worked for a large company, if we couldn't find anyone who was willing to serve as a product champion, I sometimes was able to get the project canceled, or at least postponed. If no one cares enough about it to make the project succeed, why spend money and time on it?