Some vallejo paints have a tendency to separate quite quickly, I have one or two, that, even if I paint two weekends in a row, have separated into pigment, carrier and fluid. A quick shake or stir fixes it right up.
I've been mixing the various ranges of Vallejo for quite a while, never had any problem with. Some do separate when mixed, especially when thinned or cut with a medium, but a quick swirl fixes that right up