Steve if you are using Vallejo use their thinner as it has the same ingredients as their paint & varnish.
Aidan I bet your were to far away form the model & when it hit the particles as has been said where already drying. If that happens again try thinning with the paint manufacturers thiinners not water.
Subject of needle size & pressure. If your needle size is .4 you will get a larger amount of paint rhro. compared to .2 needle size
With a higher air pressure you will get more paint delivered than a lower pressure.
Near to the subject you will get more paint delivered to a smaller area than further away which will give more sparse coverage to that larger area.
All of which I have found is that you have to experiment to find the optimum.
On top of all of this is temperature of the paint temperature in the room how much you pull the trigger back on a 2 stage brush & the type of brush & paint you use.
Other than that it is not very complicated !
So my idea. Pressure less than 20 Vallejo
Model Air thinned approx 20%. Thin "only" with Vallejo thinner. Keep temperature stable at the temp you will always try to use (summer difficult). Spray to begin at 6" form the model.
Then practice & experiment time & again. In the end you will find with time what works.
Very easy really !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Laurie