-First make sure she is not having peer / social problems at school. Some students don't want to go to school because they feel picked on, etc. and yet don't tell their parents about it. (In addition ask the school counselor to call her in and talk to her privately to get a feel for the situation).
-How are her organizational skills? Does she keep her binder together? Sometimes we falsely assume that students know how to organize themselves; however, if they've never been shown how to put together and use a system, there are some who can not figure it out on their own and are too embarrassed to admit it.
-Is her locker organized? You may want to ask her about the condition of her locker. Perhaps she can't find the papers she needs when she needs them (such as directions on how to complete an assignment).
-Have you attended any parent conferences with your daughter present? It is always best the student be there in order to say what is on his or her mind and to hear feedback directly from the teacher.
-Instead of telling her to attend tutoring (some students are embarrassed to attend), if you can afford it, consider hiring a private tutor who will come to your house once a week until she gets caught up.
-Depending on how she responds, you can try positive or negative incentives / reinforcers. (Positive such as rewards - allowing her special priviledges or toys for good performance. Other students respond best to having things taken away. If she is always on a cell phone, text-messaging or playing video games, those toys can GO until she turns her grades around).
-Sometimes a student simply isn't very interested in a particular subject; however, perhaps you can try and interest her by bringing up what she is learning on a day-to-day basis and applying it around the house, taking trips to places she studies, etc. (When my son was in 6th grade social studies he wasn't very excited about Mayan history until we finally went to Mexico and then it all suddenly came to life for him.)
Good luck, and know that you are not alone. Many parents struggle with similar situations. :-)