It has been discussed that Elena was watching Nikita because there no trust from her that he can do it. No trust - no partnership! Chock and bates already have trust that's why there already better than I/k and that's a team that didn't exist when i/k were at their jr best.
Yes, it looked as she was watching him and got distracted. But it is normal that one partner is looking the other to follow him/her, right?