Funny how a similar question was asked in another forum just wo days ago!
My comment was the following:
"I had the same problem with some bottles, maily with new ones. Even coating the pain job with matt varish had no effect, just like it happened to you!
My opinion is that it's the solvent, when the paint is not mixed well.
In fact once I shook and bashed the bottles on the table (!!) i got a well mixed paint and no more "satin-problem".
BTW the "Vallejo Model Air" line of products will always give you a satin look when used with a brush; only with an airbrush it will be matt.
I am using drinking water from the bottle to paint my figures, because is less calcareous than our tap water."
Hope this helps.