I had a site then made a forum and put an existing adsense ad from my old site into my new forum ,but adsense didn't detect any visitor. is it against adsense policy to put the same ad in two different sites with 2 different topics?any help?
No you can do that but maybe you do not have any visitors to the forum? Or the forum is to new? Or you are blocking Google in this forum. You can use the Adsense code on many site as long as they are within the terms with Google.
it's not against tos at all.. Maybe you're using the authorized sites feature in your adsense account and forgot to add your forum url, I don't know. It shouldn't happen.