Possibly different brands of paint / primer reacting adversely. Cold weather also may be a factor if you're spraying it outdoors and bringing it indoors immediately. The difference in temperatures will cause a condenseate effect in the paint resulting it what you see.
There are those few factors I can offer from pure experience.
If you can't spray indoors, then take a hairdryer or heat gun and get the can warm....shake it constantly until the temperature in the can stays stable and warm even when shaking......then go outside, shoot the peice and come back in. In the small amount of time it will take to shoot that small of a peice you shouldn't have any problem with condensation since the paint AND peice will be of about the same temperature.
Best of luck :thumbsup
Steve