Rejected payments normally occur when the clicker of the link did not receive the interstitial advertisement. This could be due to some kind of technical glitch or the recipient having some kind of ad-blocking plugin? If you click on the "i" icon on the left of each row it tells you whether the advertisement was shown to the intended recipient and whether or not they clicked through to the destination site. If they saw the ad but did not follow through, you still get paid your few satoshi.
Hope that helps.
