AnimationTrack:AdjustWeight() does nothing

I want to smoothly blend two animations. To do this i am using AnimationTrack:AdjustWeight()

The problem is, this function appears to be doing nothing.

print(AimTrack.WeightCurrent) --Always prints zero
print(aim) --Prints true or false depending if the mouse was pressed or released
if aim then 
	AimTrack:AdjustWeight(2, .3)
else
	AimTrack:AdjustWeight(0, .3)
end

I have noticed that the Play() and Stop() functions also come with fadetime parameters, and using these i have made this work the way i wanted it to, but i still wonder why these functions are not executing/only going to zero