Hi

While I agree the spring is the most likely candidate it would be worth doing a little test first. Does the gun move freely up and down if you push it gently with your finger? If so apply light pressure under the gun barrel at the end with a finger while you push the stick on the controller. This has the same effect of the pressure the spring does. If it is the spring out of its socket you will feel the gun move as it should - please just use gentle presure and don't force against this as you will damage it. If the gun does nothing it might actually be the motor or wires to it. It might sound like the motor is running but in my ones I found that the sound chip makes a noise to simulate the real thing. If this is the case it is either start pulling it apart to fix yourself if you feel confident or send it back for a replacement or refund. If the supplier will refund your return postage I would personally go with this and get another. If he won't then I would fix it myself.

I hope this helps

Gorehound