Did you wait a bit and check again? Ads don't always work right away.

thanks, I usually wait 10 minutes and it works.

For some reason this time I had to wait longer, and also deleted cookies and refreshed the page until it finally worked.

thanks,
D.P.

You may not have much advertising available on the page(s) you did this on. Adding the new add unit may mean either one ad unit or the other shows, but not both.