What can I expect from a fixative? I finished a charcoal drawing and want to paint over it with a clear coating (of acrylic gesso). For that I first used a fixative so that the drawing will not smudge when I apply gesso with a Brush. However, it seems that the fixative does not work the way I intended. The charcoal is still (after a couple of coatings with fixative) not fixed, meaning that when i slightly rub on the surface it leaves a trace.

What might be the reason for my drawing not being fixed completely?

Could it be that the fixative is not good? Do I have to apply a couple more coatings of fixative? 

Is it even possible to fix a charcoal drawing in such a way that it is unsusceptible to rubbing or brushing? If yes, how can I achieve this?
Yes, it is absolutely possible to completely fixate the pigment on its support, but there is a lot of difference in quality between brands and purposes. So, yes, you probably have a low-quality or inappropriate one - assuming your support is not wet or too smooth, which can both impede the fixation.
A couple of coatings is usually the recommended application. Apply the fixative evenly, about 30 cm perpendicular to the paper.
I stumbled upon this blog post on jacksonart.org with an extensive comparison of several brands and types, offering information and insight in the process.