'Error' makes it sound like a fault- it's porbably not so much something you did wrong as something you can do better.
Lacquers are a bit different then enamels or acrylics in that you can spray them much 'wetter' than other paints (using much more reducer). So you can build it up in light coats with almost no surface texture. You're completely right that too little thinner will allow the paint to partially dry in the spray stream, building up a bumpy texture. Spraying with plenty of reducer will eliminate this, and will even let the paint 'level' a bit on the surface it's sprayed on. You just have to control for pooling and runs by making your application coats light (no 'wet' coats!!).
One additional problem you might run into is that a very reduced (thinner rich) mixture of primer may allow the thinner to attack the bare plastic. This will often show as mold lines etching themselves into the plastic after they've been sanded off. If this happens, you'll have to spray the primer with less reducer, and accept that you will have to build up a thickish, possibly textured layer to protect the plastic first. But once you have this barrier, you can gently sand out the texture and spray additional well reduced layers to work up a perfect base for overpainting.