if you have repeat visitors, then they are already familiar with the ads on your site so they will not click. another thing can be your ads, you may have different ads now than before which is not being appealing to your visitors. what you can do is to change your ad placement or look. play around with it a little and sometimes it helps increase the CTR and revenue with minor tweaks
That's possible. Maybe the ads showing in your page is not so relevant with your content. People tend to click your ads if they found it useful and is related to the topic that they want for.
You will have to do good link building so you can get more search engine traffic and thus more clicks. Also try blending your ads with your site's color.
Yeah this is basically true. Or the visitors could be blocking the ads as they do not wish to see them.