It may be related to allowed sites feature. If it is enabled ad will only display on selected domain and it will not display on other places.
No, if you use the Allowed Sites feature, ads will still display on sites that are not allowed to show them according to your settings, but impressions and clicks won't be credited to your account.
Oh and ads are showing just fine on Google cache pages (though a little off-topic), so I suppose it makes sense there hasn't been a peep about it from anyone anywhere. Can't think of a reason why they're not showing for you though. Did you provide the other people on IM with a direct link to the Google cache, or did they go there on their own?