No, I am using backup ads and as far as I can tell they appear.

How exactly are you implementing your code?

Thanks. I set a url in Adsense and then just place the code on that page at the url. It's always worked in the past...

Have you checked the code on the page by visiting the URL directly?

Yes, the ads come right up - no problem...

Which ads are you using as passback?

I'm using Digital Throttle as my passback ads...

With the help of Digital Throttle, I've figured out the problem. At least the one I'm having. The issue is happening to me in Chrome (not Safari). I haven't tried other browsers yet but I'm almost certain this is a Chrome-related issue. At the top right corner, at the end of the URL bar, there's a little grey shield with a red X. The pages where this is happening are https and green (Secure) - but this little icon, when hovered over, states: This page is trying to load scripts from unauthenticated sources.

When I click "Load unsafe scripts" and then "OK", the ads instantly display... The passback ad code I'm using IS Digital Throttle's "secure" code - so that shouldn't be an issue. Here's the "shield" I'm referring to: [s18.postimg.org...]

Does anyone have ideas why this might be happening? This did not become an issue until recently...

It looks like MY specific "blank ad" issue was fixed by going into each of my Adsense ads and switching the passback URL to https...