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Do you always use fixative? What sort? The stuff they get in over here always sprays on in spots - and also darkens the drawing up.... so I never use it.
Linda, I always use Winsor & Newton's fixative for "Pastel, Charcoal & Pencil".
Spray in a number of light coats while holding your breath. I've smelled worse brands but you'll need running shoes to get away while your studio return to a normal Earth atmosphere!
Spitting can occur although I don't usually have problem with W&N. 3M made an excellent fixative but I stopped using it when they redesigned the spray head and it developed a tendency to spit. Because of that I now always place spare card at both sides of my drawing then spray onto one of these first. If it sprays evenly, I'll pass it across the drawing and back. This is repeated until I feel everything is glued into place.
I don't find that it darkens the drawing but fixative does has the added advantage of dulling the sheen of the pencil. I wonder if it is this that you are noticing? With the glare removed, it's often the first time I really get to see my drawing properly. I think of it as quite a magical moment 
And a good fixative also contains a UV filter to lessen the chances of light yellowing your paper.
And I've found that the W&N fixative dries with a matt finish that restores tooth! To obtain a really dense dark tone I sometimes (as in the background of my last Cairn drawing) fix the drawing then reapply graphite to the dark areas. The fixative supplies enough additional key to support more graphite. Then a top-coat of fixative stops it from dropping off again 
And (this is the AND, honest!) when you've used your fixative be sure to invert the can and spray until only propellant can be seen. This will keep the spray head clear and prevent spitting next time.
MIKE
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