There is a figure quoted (I shall have to try and dig it up) that shows the number of people applying for clearances far exceeds the forecast. This is added to by failed IT projects and lack of staff following the transition from MOD to Cabinet Office.
There is work ongoing with industry to allow the NSV to better forecast demand, and it seems they are almost there with sorting out the basic elements of their IT, but currently the system is in an absolutely shambolic state and has almost ground to a complete halt in the last 6 months.