I use proprietary products, too, to a degree.  Specifically, I found I get my best results with Tamiya's acrylics, when I use Tamiya's acrylic thinner.  I tried other things, specifically water, and isopropyl.  But neither worked for me.  An exception to this is that when airbrushing, I have tried generic lacquer thinner and that worked well, too.  That was an experiment, to see if it would enhance the matte finish of the flat colors (it appeared to).

For other media, though, such as various brand of enamels, or lacquers, I have been able to use generic mineral spirits or lacquer thinners.

But it is sound advice, especially with Vallejo's Model Air airbrush products.  If you're using a paint, and it doesn't seem to work right, then consider using the maker's thinner, retarder, etc, and see if that resolves the problem.