Ads not working on some sites

I've noticed when I see some ads on something that interests me from other sites, the links don't work. What causes this?

 

I don't know what you're talking about, give us an example.

 

Ok, lets say I visit a website and theres some interesting looking google ads on the site, I click on them but it does nothing, takes me nowhere.

 

No not all the time, just every now and then. They are not my sites, and yes just sometimes with google ads.