If you don't get good answers here, try the tools and techniques hiearchy. There are a couple of forums there (painting and airbrushing, and techniques) that frequently deal with washes.
If you are truly using mineral spirits I have no idea what is wrong. However, if you are using a generic "paint thinner", there is a new kind of paint thinner being sold that is a milky white, not perfectly transparent, and there are dissolved white particles in it. Do NOT use that stuff!