I have had countless problems with pastel fixatives and I have totally ruined many a good painting by using too much of the stuff.
The first solution here is to use as little as you can get away with. Even the absolute best quality fixatives will tend to have a slight dulling effect on the colours and, if you use too much, you will make the surface "frosted" - or even shiny. (And it's quite difficult to apply pastels to a shiny surface!) I want the fixative to bind the pastel particles sufficiently to be able to add a subsequent layer - and nothing more. For this reason, I never use fixat