actually it's probably down to the curvature of the filament.
The second half of the roll will be more tightly wound than the first half, so the filament will have a natural 'bend' or curve to it that gets stronger and tighter as you get towards the core of the reel.

Trying to push this through a bowden tube will result in increased friction the further down the reel you go.

So not only does it take more power for the motor to pull it from the reel, it also takes more for it to push it through the tube.
The two combined might overcome the steppers motors torque.

Depending on your firmware you might be able to allocate more power to the extruder stepper motor - I know you can do this in marlin.

As far as moisture goes, I find that only the outer couple of layers of a reel that's been left standing is ever effected. So it's unlikely to be that.