It sounds to me like she's having the same issues with spelling and English that my son had last year when he was in 4th grade: Laziness. He did excellent in his other subjects, but he wouldn't study his spelling and grammar.
For us, real life consequences were the best. He "hates spelling," and "doesn't care" if he fails, he'd say. I told him quite simply that if he didn't get at least a B on his spelling tests, he would lose his iPod Touch and all TV for the week. The very next spelling test, he brought home an A.
I never harped on him or fought with him over homework. I told him that his grades and homework were his responsibility, and that if he brought home bad grades because he chose not to put in the effort, there would be consequences.
He tested me a couple of times by bringing home C's. And each time it was obvious that he hadn't studied. I followed through on my promise to remove his favorite things until he brought home an A or B, which he always did the following week. It wasn't lack of understanding, it was lack of effort that was his problem.
Figure out her "currency." Find what she absolutely loves. And then, tell her that if she brings home less than a B, she loses that thing. Then back off and stopp nagging. Give her the responsibility to do HER job, and follow through on yours.
Lastly, and this is most important in an academically focused home like yours (and mine): Be sure you celebrate the triumphs. It's easy to get focused on the bad stuff to the point that the good stuff isn't recognized. So be sure that you mention how proud you are of the hard work that she's put into those A's and B's.
ETA: I just wanted to add, taking away the things he loves never made him resent his schoolwork. The mantra in our home is: We do our work before we go run off an have fun. We don't procrastinate.
I don't especially enjoy some of the responsibilities I have, but I do them first knowing that I can relax or have fun when I'm finished. That's a real-life lesson that all children need.