Airbrushing is a triangular procedure in that there are three things that affect the finished job (5 if you include the airbrush and operator). They are 1) Paint consistency, 2) Air pressure, 3) Distance from the surface. To get a good paint job they all three have to be balanced within a relatively wide range. If your paint is too thin or the pressure is too high or you are too close to the surface you will get a runny finish. Likewise, if your paint is too thick or your pressure is too low or you are too far from the surface you'll get speckling.
I usually thin my paint about 3:1 (paint: thinner), spray at about 15 psi, and spray from about 3-4" from the surface. These will vary from airbrush to airbrush but should be a good starting place.
Humbrol is good paint and enamels usually spray very well. It sounds like yours is thinned about right so I'd suspect that it's either the perssure or your distance from the surface that is causing your problems. Try playing with your air pressure and see if that helps. Also make sure your airbrush is thoroughly clean and you don't have any air leaks along the hose anywhere (spray some soapy water on all the connections and look for bubbles).