Is the small spring between the trigger assembly and the tappet plate still there?
When I changed my internals to a new V2 mechbox shell, I took dozens of photos as I was removing the selector plate and trigger assembly. It helped a lot to put everything back in place. I unfortunately deleted them since then.
If you can take a few close up photos of the trigger assembly, maybe someone here will be able to see what's wrong.
You can see some photos there, but only 1 or 2 show the trigger assembly:
http://www.airsoftretreat.com/featur...mechbox_guide/