Nony,
If that is the 16, I had a similar problem. Turned out to be the retract units themselves. The three screws per side that sandwiches the trunion into place I found on one to be too tight. A little less pressure solved the problem. I just backed the screws out slightly and then loctited so they don't move. This relieved the pressure on the trunion making it move more freely.
Mark