The paint is drying before it hits the styrene. 50PSI is WAY too much pressure for airbrushing. If you are using a gravity feed AB drop the pressure to 12-15, and if siphon feed try 15 to 25. Note that those are ranges only. Internal mix brushes usually need less air, while external mix ones require a higher pressure.
So less air, and get up closer to the plastic. No more than 6" away. I typically spray around 2" to 4" away and never more than 15PSI using my Badger 150 siphon feed. When I use my Badger 100LG I can be as close as 1/2" and as low as 8PSI.
Another thing to take into consideration is your thinning agent. Tamiya thinner is basically Isopropyl Alcohol with an acrylic retarder added. If you are using straight Isopropyl, it can dry very fast indeed. You can buy the retarder at art stores like Michael's.