This black rubber cork-shaped piece with a metal disk and pin is a loose part, meaning it may come out from time to time. Do not worry because it is easy to re-install it.
Simply push it back in place with the metal pin facing up. Alternatively, you can push the rubber cork side into the glass vial (pin facing upwards) and then push it back into the foreshots collection area. Always make sure the cork and vial are installed for it to fill with foreshots. If the cork is not in place, but the vial is inserted, it will not fill with foreshots.