I had this same problem. Well, actually after I modded it. I hope you are good with your hands, because this takes effort.
Unscrew the casing and open the gun (remember to take the battery pack out and have the two plates together so that the wire connecting them doesn't break).
When you see the plating covering the plunger (spring) system take it off, it has about 5 screws.
Since this is extrememly hard to explain in words, look at an internet picture of the internal plunger system. The way it works is that when the spring/tube goes forward (as the motor operates) the 'plunder' (the white thing) stays back. As the orange tube goes forward it triggers and release mechanism that fires the plunger forward due to spring pressure, compressing the air, therefore firing the bullet.
All you have to do (which is what I did) is insert a clip (with the plates open), make sure the jam door is closed, and put the battery pack in (to complete the circuit). Do a bunch of test runs to make sure the white plunger is shooting forward as the mechanism operates.
This will all make sense once you activate the gun without base plates.
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if you complete it, have questions and/or comments.
I hope it helped!
PS- while you're at it, you might as well look for online instructions on how to mod the air restrictor (as I am too lazy to write a few more paragraphs!).