Has the number of leaking parts always been consistent, or has it recently gone up? Is there a process in order to clean the pins prior to molding around them? If the pins are clean and free of oil or other contraband, it should mold without leaks.

However, there's a lot of missing pieces. What type of pins? Are they specifically for over molding? Are you drying the material to be sure there's no moisture? Just trying to think of reasons why the bond would be good on some and not others.