text: Outsider, has pay risen over the years? 
  https://nces.ed.gov/programs/digest/d13/tables/dt13_211.60.asp
  
  This gives salaries (in current dollars) since the late 1960s (roughly once per decade plus the past few years).
  
  54,006	48,474	56,191	57,856	58,948	58,302	58,758
  
  The change in salary can't possibly be the reason for the shortfall, can it? Maybe more people being paid but fewer interacting with students could do it. Personnel costs others than salary could do it (health insurance, retirement, PERS charges). But salary doesn't seem to be the driver.