Did you by any chance go back and revise the listing in any way?

That can lead to a delay.

The most likely explanation is that is is simply 'in a queue' to go live on the site.  It can take 24 hours (or more) for listings to be visible.

As soon as you list, you are told your listing is live but not all items listed together appear at the same time.

For now, do nothing.

Check again tomorrow morning and your missing listing will probably be there.