You should always wash all parts to be painted with mild dishwashing soap and use an old, soft toothbrush to get into all of the nooks and crannys. This will get rid of any left over mold release agent left over from the factory when the popped the part out of the mold. Failing to do this can cause the results you're having. Use a soft lint free towel or shammy to dry the parts. Avoid touching the parts with your fingers as the oils from your fingers can cause issues too.
Next, old paint or paint that is not well mixed cause poor results. Avoid paints that are on sale due to excessive shelf time at the store. Always mix (shake) your paint very well.
As stated earlier, warming the can will help the paint to better atomize as it leaves the nozzle. Fill a bowl with quite warm water, not hot (remember, this stuff is under pressure and can explode if too hot), and stand the can upright in the bowl. You do not want water to get near the nozzle area if you can avoid it. If this does happen, you can dry it with a towel before you shoot. Let the can sit in the water for 5 minutes or so. This raises the temp of the contents of the can which in turn raises the presure in the can. Also, warm paint flows better then cold paint. Primer can be done the same as paint when it comes to all of this and the use of primer will give the paint more tooth to grab onto so you will achieve better results.
When shooting paint, I usually shoot a couple of very light mist coats at first. You just want to tint the parts with the paint. Again, this gives the following coats something to grab onto. The following coats should always be light. Remember, it's a lot easier to add more paint then it is to remove it!
Try to shoot when the humidity is low and the temps are in the 70's or 80's if possible. Too cold and the paint will not level out on the surface before it sets up. Too hot and it takes so long to dry that airborn dust and stuff can be picked up due to longer drying times.
Lastly, a polishing kit can help really bring out the shine in the final coat of paint.
