Im not sure exactly what the deal is, but if the machine is operating correctly, you should be able to spool wire any time by pressing the trigger on the gun.
What is the wire speed set at? Open the door and check to see if the drive rollers are turning. You should see the bottom roller with the groove in it turning when you press the trigger. If the drive rollers go and the wire doesnt spool, you could have a couple things going on.
Feed the wire by hand all the way out of the gun. If you can feed it through easily thats good and means there istn't a problem with the gun or cable. Next check the tension on the roller that presses the wire into the drive roller. If its loose the drive roller wont get traction on the wire and it will spin out. The other thing to check is that the nut isn't putting too much drag on roll of wire.