Adobe Illustrator: Unable to apply Grain-Effect to Gradient

(This question is related to another investigation into visual techniques using gradients and noise-filters: Research: Grainy/Noisy Gradient)

I am trying to apply a Grain-Effect (Effect -> Texture -> Grain) to a shape with a gradient as its fill color. Although this has worked with other shapes and gradients before, here the Grain-Effect somehow is not displayed the way it usually is. It is basically not visible, the shape remains filled with a smooth gradient. Take a look at these screenshots to see what I am referring to (in this example, Shape A refers to all the other shapes of my current project on which the effect has worked without any problems).

1. Shape A and B before the application of the effect (+ their respective gradient-settings):

(Gradient Shape A)

(Gradient Shape B)

2. Grain-Effect settings while adding the effect to A/B:

(Adding the effect to Shape A)

(Adding the effect to Shape B)

2.2 Closer look at Grain-Effect on Shape B:

A closer look at the shape while applying the effect reveals that some grain/noise particles are somehow still added to the shape's fill.

2.3 Shape A and B after the application of the effect:

Does anyone know why this shape/gradient is behaving this way? I would like to be able to apply the Grain-Effect to it just the way I'm doing it to my other shapes, i.e. Shape A in the example above.