This was completely and totally me.
Part of the problem was that I was bored through much of my elementary education- through the later months of 6th grade- becuase I had seen so much of the information before and knew what it was. We were often in the after-school care program and studying was impossible. So I didn't study. I had to take all my 6th grade final exams (and tests that I'd be missing) early for an international swim meet that overlapped. All of a sudden, I had to study becuase I had never seen this material before in the classroom or in my rather nerdy habits (ie, I read anything... and retain even what I don't want to!). I did really, really well. So, at the start of 7th grade it was a, "Huh? Since I haven't seen this and they're expecting a lot more of me now, what will happen if I study?"
I went to boarding school for high school and was accepted to Ivy League for college, although I chose to take a gap year and married and have attended schools in the areas we've lived. I am at the University of Maryland, College Park, right now and
love it. I am challenged and work hard, but it is not the point that school runs my life. Because my major is so small, I know there are students that do really struggle with some of the classes. So, for me, I think I do as well as I do based on the fact that I do have a very broad knowledge and experience base and have been able to identify which ways of studying work best for me. The work habits that were ingrained during high school have only helped me to gain success and manage my commitments.
DH is in business school at George Washington and is frustrated that he has to do so much more to maintain a grade point average much lower than mine, even though it seems that I put less into each of my classes than him. No questions asked, I am smarter than he is (and he has no problem or qualms with this fact-- his was the Division 1 jock

), but he does well in
his own way. I try to remind him that it's not a competition between you and anyone else- it is what you can and are capable of doing given a series of contexts and circumstances.
If things are going as well for your daughter as you think they should, given the amount of time and effort she puts forth, try studying a different way. Maybe she needs to hear the information- you read it aloud to her and she repeats it; or she simply recites it aloud again and again. Maybe flashcards are right up her alley. I really prescribe to Gardner's theory of multiple intelligences. Maybe take a look and see which ones best suit DD and try making slow changes to really focus on her strengths. And, if her best is a B average and that's what she has, there is
absolutely nothing wrong with that. She's doing the best she can and putting forth the energy and time to accomplish that! And when it's your best, it's your best.
ETA: It could also be the way she's tested. For example, your daughter might excel for short-answer and essay questions and really struggle with multiple choice or fill-in-the-blank formats. Unfortunately, essays aren't really used until later grades and high school as a testing tool.