Once you change the orings in the flow knob...........put it in the on position and NEVER touch it again. Those orings wear out very easily and if you use the knob, you'll keep having problems. Turn your air off at the ASA if you must.
Or get a CP plug and lose the knob totally.
I have no idea what you mean when you say you tightened the plug to fix the leak? That makes no sense to me. It rotates and can't be tight. Unless you screwed the stop screw in too far and it's hung up on the metal of the knob.
Now, since the gun fires once then stops....are you sure you're turning the knob the right way? If you have it in the ON position when you air up.....then turn it into the OFF position before you fire(thinking you turned it on), you'll have about one shot of air in the gun. Then nothing because you turned the flow off.
Second. If you have a dead or old or dud battery in the gun.....there may only be enough juice to cycle the solenoid one time or half a cycle. Try a different battery of a good name brand. Not cheapos.
It's all about the angles.