thinning enamels

Wow! I have been airbrushing Humbrol for years and never had any such problem. When you say you are using Mr Thinner for thinning, are you referring to the Gunze product? I have never tried that combo. I have used plain hardware store paint thinner or Testors airbrush thinner. And never had any problems.

yes the gunze thinner!

I would suggest trying Humbrol thinner at the very least. I do not know of the compatability of Gunze Mr Thinner with Humbrol Enamels. I know it works with their stuff, both acrylic and lacquer, and some acrylics but not others. If you are using Humbrol acrylics, try distilled water, denatured alcohol, or winshiled washer fluid.  

As stipusher said: Use Humbrol Thinner or any other brand especially for Enamels, like Testors Thinner. WHite spirit would also do the job.

I know that too high a pressure can cause paint itself to dry too fast and go on very rough, and gloss paint to go on flat.  What pressure are you using for your spraying?  I don't know if too high of a pressure can cause it to dry too fast in the nozzle, but I suppose it is a possibility.

Considering that Mr Color thinner is a lacquer thinner, it may simply be too "hot" for the Humbrol enamel and may be evaporating too fast. causing your drying problems. As suggested, try simple Mineral Spirits which is essentially the same as the standard Humbrol thinner.

Having said that, if the only way to get  paint flow with your current thinner is to mix it very thin, you may wish to try that in combination with a lower air pressure to gain a little more control.  It may take more coats to build the desired colour depth, but multiple thin coats is preferable to a single heavy coat.