I have found some generic paint thinners really cruddy. By that I mean they have dissolved solids in them. They are a milky white in appearance. So now I hold out for cans labeled specifically as mineral spirits or turpentine. Since I began using these two materials I have had no real problems.
I will say the stuff sold by Testors as "airbrush thinner" gave me the best results, but I find it hard to find that stuff any more, so I have resorted to hardware store thinner. I find it works okay, just not quite as good, especially on gloss surfaces.