It could be a number of things:
Cool weather. Are you using the heater much? I think that even without the heater, range will be reduced in cold weather.
Change in driving style, maybe driving at higher speeds.
Change in usual driving routes. I get about 64 miles of range this time of year, but I think that my route contributes to the low number. I usually start out driving downhill with a full charge and then drive back home uphill. Going downhill with a full battery, I don't get much benefit from regen. The battery seems to be OK, though. About 16 kWh according to the energy display, which I think is pretty normal at 18K miles.
I'm sure someone else will be along with a more detailed answer.
2015 Spark EV 1LT, DCFC