Airbrush/Paint problems

My first thought would be insufficiently thinned paint. After that, I would be looking for an improperly cleaned airbrush. If you hadn't mentioned replacing your needle and nozzle, I would be looking for a bent/burred needle or a damaged nozzle. If your paint is sufficiently thinned, I would be looking for an accumulation of dried paint inside the airbrush.

Mineral spirits should work successfully with MM enamels. It does with other brands too.

Generic "paint thinners" can vary from straight mineral spirits to the next nearest thing to lacquer thinner and your results may vary from brand to brand.

Yes!  Many hardware stores are selling a paint thinner that is not what we used to think of as turpentine or mineral spirits.  The best thinner for enamels is turpentine, but mineral spirits is a good second best.  Do NOT use any of this new stuff, which is milky white, not perfectly clear. It is junk. It has a lot of insoluble particles, does not thin testors well, and even effects the color of some colors.  Many hardware stores sell both turpentine or mineral spirits, the latter being a bit cheaper than the former.  The new translucent stuff is cheaper than either- I guess why it is becoming so popular.