I had a issue where I didn't feel it was biting when I put mine in on the front and it wasn't getting tight, possibly same as you. I only got the screw in the front of the plastic retainer but I missed the back hole, b/c I couldn't see it. I took the screw out and readjusted the plastic retainer b/c I was trying to put the retainer over the metal piece and the plastic piece, making the retainer spread out too far and not allowing the screw to get in line. I moved the retainer to be over the metal only.
I swapped in a fresh retainer and ended up using the spread out retainer on the rear(the spot where you needed to use the plastic spacer and quicker to push on it).
Dave