In the midway of the 12th episode the #35 Diclonius failed to use her vectors. Also just before she failed to do so, she was attacking Nana or #7 at the edge of the bridge. The #7th saw something at the #35th's chin/neck and one of the #7th's vectors attacked and took #35 by surprise. Soon after the attack the #35 was no more able to use her vectors. So my question is what did #7 do to stop #35 from using her vectors? Because nothing was shown as to what was #35th's weakness and how did #7th find that weakness.
The vectors are unable to be used when the Diclonius is in pain. That's why the scientists keep them in pain. They won't be able to use their vectors to escape. The reason why #35 can't use her vectors after #7 hits her in the head has a lot to do with this weakness. Maybe the blow affected her frontal lobe in some way.