This is usually caused by improper triggering technique. To do a line , push down for air, start moving in your desired direction, then pull back for paint, at the end while moving slowly move the trigger forward to stop paint flow, then let off the air. This should give you nice lines with a bit of practice. A lot of things in the beginning are pace and technique. Training your brain and hands to do everything in the correct order.
if this doesnt solve your issue. Break down your airbrush and clean and inspect the nozzle. May be some old gunk stuck in the nozzle not allowing the needle to fully seat. Be gentle with the nozzle they can crack and give you all kinda if headaches.
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