What kind of paper/support are you using? Is the painting being
jarred, bumped, or otherwise jostled?
Pastel is a very fragile medium, and it doesn't take much to make the
pastel dust fall off the paper. If you're using a paper with a lot of
texture, the pastel might hold very well in the "valleys" of the
paper, but be somewhat looser on the "hills."
Other than that, I can think of no reason why the color should
separate or change once it's laid down. Pastel has very little
binder. Rembrandt is a reputable brand, so the quality should be
good.
-pd
--
Peter Davis
http://world.std.com/~pd
"If you can't behave yourself, who can you behave?"