interstitial works after unchecking 'Out of page (interstitial) ad slot"

We've been trying to use interstitial ads throughout a website, in this particular case to create a wallpaper ad. Initially I setup the ad, checked the interstitial box, configured the display using the Context module. I could see the code being put in the top of the page, just after the body tag, but it wouldn't display.

After a lot of tinkering I discovered that if I uncheck the "Out of page (interstitial) ad slot" checkbox after it's setup, the ad displays properly.

If I want to change the context configuration I need to recheck the "Out of page (interstitial) ad slot" checkbox, go to the Context page, make changes, return to the config page for the ad and uncheck the "Out of page (interstitial) ad slot" checkbox again.

Screenshots that show a working configuration are attached.

Thanks in advance.