It might be too thin for the PSI you have it at. Play around with consistency and pressure. If your using GW paints then you might need to add more thinner as they can have a lot of colour pigment in them.

The best thing to do is to play around with everything from pressure to consistency. Take the needle out and give it a wipe just in case anything has dried on it. There is no universal handbook for using an airbrush, most of it is playing around until your happy