ModelMan8,
The running could be due to a number of factors. How much did you thin the paint before you airbrushed it? What pressure did you have the airbrush set at? How thick did you apply the paint?
Ideally you need to have the paint thin enough to spray smoothly without clogging, but not so thin that it runs. The consistency of 2% milk is often quoted as ideal.
For pressure, it's good to have it almost dry when it hits the model, but not so dry that it is pebbly.
All of these things can affect the paint's performance. And that doesn't include outside variables like humidity and temperature.
Hope this helps.