Do you mean that she had discovered the mistake earlier, and at that time did not try to find the person responisble (in this case you are correct)...but she had not tried to find the particular responsible person.
Or she discovered when you called that someone made a mistake, and at that time she didnt try to find the person responsible. In this case you should say:
"...but she did not try to find the particular person responsible."
The first bold part of your text is fine and would not change in any case.
Student or Learner