One such situation is when the situations are complex, for example economical and the media don't report on it because they fear people won't be able to follow it. Another situation is that often positive situations are not reported, since they "don't sell" as much.
Further, Media can sympathize with one side and be biased towards this side.
Finally, Media can not tackle some problems (human rights abuses far away) because they feel it won't sell as many papers.