Could be several things.
The paint could be drying between the nozzle and the surface. You can help this by spraying at a lower pressure or closer to the surface of the model. You can also add a drop or two of acrylic retarder to your paint.
You could be getting "Dry Tip". Look for buildup on the tip of your airbrush and anytime it's there wipe it off. Acrylics are notorious for this.
The fine nozzle may be too fine. Not sure what airbrush you have, but from what I've read some of the finer nozzles are just too small for the pigments in the paint we use.
You may need to thin the paint a little more. Finer nozzles typically need slightly thinner paint.