Google ads don't show on some pages

What would appear instead if not google ads? Maybe you've embedded a wrong adsense code or no bids for keywords that are present on that specific page.

When theres not enough content on the page you don't see ads. Or there maybe no ads for the keyword.

if your ads were there before but not there now then you maybe part of googles reshuffle our site ads are now normal but we had public service ads or no ads for a while

I think its on the template I guess I have tried in once I add an ads and it did not work out then I try another the it appear. I know the codes are right because I only copy the code from Google adsense.