This is our second year of red wine-making. Last year after primary
fermentation was complete (@0 Brix), the wine was put into carboys and
there was very aggressive CO2 bubbling coming through the air locks
for some weeks. This year we did everything pretty much the same
except this year almost no action at all in the air locks from day one
onward. SO2, TA and pH all look good and the wine tastes fine at this
stage.

Why the difference? What would cause little or no CO2 activity?