While the media, or fourth estate, sometimes functions as a whistleblower, shining a light on government corruption (as in Watergate) or publicizing negative public reaction to government policies in a way that blocks those policies, more often than not, it does what Noam Chomsky in a 1988 book called "manufacturing consent." Therefore the media is often not an effective check on government.
The media is not the most effective check on government because the government courts members of the major media outlets, giving them special access, inviting them to dinners and events and often feeding them "scoops" in forms of "leaks" by top government officials. This is hardly done naively: top government officials strive to shape the narrative that appears in daily papers and on daily news outlets to garner the most public support for their initiatives, whether these initiatives are good for the government or not. This can be seen, for example, in the repeated press narrative about needing to "save" Social Security by cutting benefits, even though the program is actually in no fiscal trouble that cannot easily be solved without benefits reductions and is a program most Americans want to leave untouched.
Furthermore, these same government officials often punish newspapers and reporters who do not toe the proper line by pulling access or hiring public relations people to react strongly and negatively to any reporters who do not cover stories the way powerful elites want them covered.
Major media outlets are also profit driven. In recent years, the internet has eroded traditional print media profits, so many news outlets have cut their investigative reporting budgets, meaning far less of it is being done.
Finally, "terrorism" has replaced "communism," Chomsky says, as a buzz word or "social-control mechanism" which the press does not challenge, leading the press, for example, not to sufficiently question the US war in Iraq.
There are at least two major points of view on this issue.
One point of view holds that the media are indeed an effective check on government action. Mass media outlets continually challenge the government on various issues. They "break" things like major government scandals. They put pressure on the government to justify itself when it does things that may be controversial (for example, there have been recent articles questioning the legality of targeted killings of suspected terrorists through the use of "drones").
Another point of view holds that the media is too closely tied to the government to truly act as an effective check. This school of thought would point to such things as the fact that media is dependent on government sources for much of its news. They point out that the media will not truly challenge what government is doing because they fear losing their access to insider sources.