What is the problem with my airbrush?

Are you shooting metallics? Some metallic paints (cough..Tamiya..cough) will do this. The metallic particles settle quickly to the bottom of the jar or cup and get sucked straight into the brush and block the paint passages. Releasing the needle pushes the paint forward and a small amount hits the air stream and <splat>.

If this is the case, the only solution is to fiddle with your thinning ratios so that the paint isn't too thick to flow nor too thin that the paint particles settle out of suspension too quickly.

No, just standard colours and a variety of brands with Vallejo being the most recent culprit.

In that case, perhaps your air pressure is too low? Dial up the pressure a bit and see if it starts to spray.

I'd also consider giving your brush a thorough cleaning before continuing.

Thanks for the advice I'll give it a go.

Also, be aware that if you are using a variety of acrylics, you should be careful of cross-contamination between thinners. The thinners for some are not compatible with others. eg. alcohol or Tamiya's own X-20A thinner can often cause clumping in Vallejo paints, which often causes an instant blockage of your airbrush.