It only happens when the can isn't properly mixed. Are you shaking the can for at least a minute and clearing the nozzle between spray applications?
I've literally never had this problem with Testors. I did do something similar with a matte spray once, but I wasn't clearing the nozzle.
Also, never use the bottom 1/8th of your can. It's not an even mixture at that point. Depending on the angle you spray, you generally use more paint than propellant, so the last bits of product in the can isn't really usable if you're going to obsess over the finish, like we all love to do.