Start by reducing your variables. Make sure your paint is mixed exactly the way the manufacturer recommends. Back when I was painting cars Sherwin Williams even supplied a viscosity cup. You'd thin the paint and then fill the cup and time how fast it emptied out of the hole in the bottom. The cup looked like a big soup ladle. If the paint ran out too fast it was too thin, add more paint. If the paint ran out to slow, add more reducer/thinner. Now you just eliminated a variable.

Manufacturers also recommend fluid nozzle sizes and pressures, so that would narrow down more variables.

The final variable was the skill of how the painter applied the paint. Speed of application, distance from the surface and amount of overlap all play an important role. Let me tell you I could paint two exact cars, the exact color mixed the exact same way, with the same equipment just two days apart and I'd still have to vary my technique between the cars to some extent.