Has the weather changed? Maybe you are getting used to driving it and not paying as much attention to economy driving. I just got a 2013 and my daughter had been getting 34-36 per charge. I just got 41 on my first charge. I reset the climate to econ and used my old Mobil Economy Run skills. I'm going to try for 45 by working on my regen braking habits.
2013 Volt Premium - Free charging at work free carpool lanes.