Social Security's Office of Inspector General (OIG) has issued a report on the agency's initial disability claims backlog. In 2010 Social Security established a goal of getting the backlog down from about a million to 525,000. That goal hasn't been achieved. The report is too polite to give the main reason, the fact that the House of Representatives passed from Democratic to Republican control after the November 2010 election. This cut agency funding and eliminated most Congressional pressure to reduce the backlog. You can see below what has happened.
Staffing had gone up significantly between 2008 and 2010. Social Security was poised to work down the backlog. They made a little progress on the backlog in 2011 and 2012 because staffing levels were still above the low level of the Bush Administration but by 2013 progress had pretty much ended as staffing levels decreased.
One reason for slight optimism about backlog levels is that initial claims receipts reached their peak in 2011 and are now declining, as more and more of the baby boom generation reaches full retirement age. Yes, despite what you may have heard, fewer disability claims are being filed, not more.