The OIS has been advised by the Department of State that the consular systems experienced technical difficulties beginning in mid-July. The problem is world-wide and not specific to any particular country or visa category. As a result, this has prevented U.S. embassies and consulates from issuing passports, visas, and other official documents. The Department of State is working diligently in investigating the cause of the performance issues and to restore full functionality to the global database.
Please refer to http://travel.state.gov/content/visas/english.html for regular updates on the status of visa issuance.