The days i have heard many peoples facing this problem in which only Public Service Ads are Appearing on your Website.

This Problem Mainly Happens When your Website denies access to search engine robots or Bots. This is the reason why Bots are not able to access your website. So the website doesnt get indexed in the search engine .
When you add the Adsense code to your website . The Google adsense crawlers try to scan the website contents and keywords upon every refresh so that it can display the relevant ads on that particular ad unit. But when your website is blocked for search engine access , the crawlers are not able to scan the page . As a result it doesnt get relevant keywords and so Google Public Service Ads are displayed on the particular ad unit .
If you are using Wordpress then you require some options to be enabled in order search engine to be able to access your website .
Under Settings Tab –> Privacy settings
Here you can see the options
Now you can Enable Blog ’s Visibility for the Search Engine .
I guess the problem gets solved here , If you have any Queries Do ask them .
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Yes indeed that is true