Armor painting

And, if you ever start to use aluminum or a metalizer paint, stir till your arm falls off.

Reid

I agree with the others, its all down to the mixing. I was recently given an electric paint stirrer, and that really does the job a lot better than shaking or stirring by hand.

I use Gloss enamel paints and sometimes find i get a semi gloss finish if it hasn't be stirred right.

I agree that mixing very well is more important with flat paints.  Also, it is possible to get semi-matt/semi-gloss with either flat or gloss paints.  Gloss paints put on very dry/thin will give a semi, as will flats put on very heavy/thick.  Try thinner coats.

Also, if you are airbrushing flats, try a slightly higher pressure and a slightly greater brush-to-model distance.

I would be more inclined to think you are not mixing the paint enough when you first use it.  Shaking the bottle isn't always enough, I use a small screwdriver, also called a tweaker by some, to stirr the paint to make sure I get all the pigment that might have settled on the bottom, then shake it....