Here are some suggestions:
1. Have you defeated Team Flare Aliana at the last part of the Power Plant?
2.You can only enter through Route 13
3.Did two people come and restore your Pokemon and give you full restores? If not,then you haven't defeated Team Flare yet.
4.Here's another option,try going to the room where the hostages are kept and talk to everyone there
5 Here is a full walkthrough of the power plant https://www.youtube.com/watch?v=KVRWLPRsGRo
If I haven't answered your question,please comment below
Hopw I helped!