Lately, many Blogger users have been complaining that AdSense ads stop showing on their blogs once they add a custom domain to their blog but shows if they revert back to their .blogspot address. Well, you are not alone. That's how the Google Adsense system now works.
If you signed up for AdSense through an AdSense host partner (eg Blogger or YouTube), ads generated from such AdSense account will not display automatically on a custom domain (aka Non-host partner website). Ads will only display on the host sharer - YouTube, or blogger.
If you would like the ads to show on a custom domain, you will have to apply for approval and why you wait for the approval, you MUST not switch back to the .blogspot address.
How To Apply For Custom Domain Approval
==> Log in to www.google.com/adsense
==> Click the gear icon in the upper right corner then, from the drop-down list, select Settings.
==> In the sidebar, Click on "Access and Authorization" > "Site authorization".
==> Tick the "Only allow certain sites to show ads for my account" box,
==> Enter the URL of your custom domain, then click "Submit".
Note that your ad units will remain blank until your request has been approved. If your application is not approved, be assured that ad serving on your host partner site will not be affected, hence you can then switch back to the .blogspot address.
DO NOT switch back to .blogspot.com while AdSense is reviewing your custom domain. If you do that, AdSense will see a domain registrar's parked domain page when they try to access your custom domain and AdSense won't approve that.
When they access your domain, they have to see your blog, so leave the blank spaces the way they are until until your application is approved.
If you don't want to wait, don't use a custom domain name.
I hope this helps.
Source: AdSense Help