First, make sure that the coating has been properly thinned. If it has, then check the travel of the needle. If the needle does not completely clear the nozzle opening when the trigger is depressed, slowly loosen the packing nut until the needle completely clears the nozzle opening when you pull the trigger.
If the gun still won’t spray, completely disassemble the gun body and clean all passages with the cleaning brush. Make sure that all of the holes in the air cap, air flow ring and the area surrounding the nozzle threads are clean and free of dried paint.