There are several threads in the Google AdSense Help forum and other forums like the Warrior Forum with some AdSense publishers complaining their AdSense ads are not loading.

The issue seems to impact new publishers, recently approved, for days and weeks after they received their official welcome message from Google.

The Google AdSense team emails the publisher:

But even days and weeks later, their ads are still not showing.

Supposedly this is a known issue. A top contributor in the forums said the team is aware of it:

I am not sure on an ETA for a fix.

Forum discussion at Google AdSense Help and Warrior Forum.