While all individuals applying for a U.S. visa are screened before the issuance of a visa, certain individuals may be subject to further screening or clearance, commonly know as Administrative Processing.
Issues that may trigger an Administrative Processing or security clearance delay include:
If a security advisory opinion is requested by the Consulate (Administrative Processing) your visa will not be issued until the clearance is received from Washington D.C. On average this takes about 60 days, but can take longer. There is no way to expedite this process nor find out the status of the case. Some U.S. consulates maintain a list on their website of cases under review. Look for Administrative Processing Check Status.
Once issued the clearance can be (at the discretion of the embassy) valid for up to four years for F-1 students and up to two years for students and scholars in J-1 status as long as there have been no substantial changes in your academic or research program.