as long as the paint you are using is oil based you can use thinner to make it flow better
as a rule of thumb, the more thinner you ad the glosier the paint will become so keep that in mind
when working with water based paint (acrylics) you can ad water
one more thing, does your airbrush have different sized nozzles ?
because if it does, that might be your problem.
i would use the medium nozzle (that what i use on my airbrush and i run tetors lacuers traight through it)