[hot_shot under fire]

Thanks for the quick replies!

I assumed one of those 2 options was the issue, I just wasn't too sure whether my limited 'Airsoft Mechanics' knowledge was sufficient enough to diagnose the issue, so I came here to be safe.

I attempted to push the motor down with my thumb and the mechbox cycled without an issue. After putting the motor plate on again it wouldn't cycle. The issue MAY be that my positive motor connector is disconnecting when I put the motor plate on, although I don't think that is the issue.

As far as the shimming goes, I didn't touch the shimming in the gearbox. If I recall correctly I put all of the shims back where they came from (As they came off with the gears when I pulled the gears out)

Perhaps I should take the M4 to a proper gun doc and get this issue resolved.