Maybe you broke the antenna contact clip in step 6 of this teardown -- that would explain your poor signal.
Okay, I found your part -- sorry for the bad pictures, I really need to get a tripod.
Look at this guide, step 14 -- the cyan/turquoise screw indicated at the top (the 4.8mm screw).
Now, look at my crappy picture. I flipped the shield over, and you can see that underneath the shield, your metal piece is supposed to be attached (welded? glued?) to that black piece, so that when you attach the black shield, it connects the 4.8mm screw (electrically) to that gold pad to the right of the screw hole. This probably explains your bad antenna performance.
I don't know how you got the piece off the plate -- I pulled at mine and couldn't get it to come off -- but you might need to glue it back on to the bottom of the plate before reinstalling. Good luck!