CYMA G18C AEP Will not fire with slide on

[lf16]

Hi guys
need a little help with my CYMA g18c. Went to play with it on Sunday and when loading up to begin a game i noticed it would not fire with the slide/cover in place.
Upon removing the slide you can fire the weapon flawlessly.With the slide on it feels like the trigger does not pull back as far. The only thing i could see possibly interfering with the trigger mechanism is the selector switch which has a bit of lateral play in it relative to the slide. Unfortunately I don't have a Allen key small enough to get out the grub screw that holds it in place to test my theory.
If any one else has had this problem or knows the workings of these any help is greatly appreciated.
cheers guys