If she has had trouble for this long, it is time to find an outside tutor.
I am in the Plano ISD and we are not that fond of Sylvan or Kuman for tutoring purposes to help children catch up. Sylvan and Kuman are businesses and their bottom line is profit.... not your child's grade. Granted they are decent for helping children, just not preferred for one on one because we teach a certain way and students are expected to learn the way it is taught by their district. Some of the outside businesses are good but they don't all teach the way the school district teaches and in the end, the student ends up confused.
The best way to find a one on one tutor is to go through your school's guidance counselor. MANY teachers tutor on the side for extra money. They usually meet students for an hour at the local library or another quiet place to work one on one. This typically is $50 an hour and well worth it.
Tutoring is also offered in a small group setting at school for free. Make sure you take advantage of that as well. You are already taking advantage of that and this is a good start. Simply working on worksheets at home will not "teach" how to do things, it is just busy work. You need someone to sit down and explain each step of the process in order for your daughter to "get it".
There is nothing wrong with hiring a tutor from school. We hired tutors for our daughter when she was in elementary school for math and writing. She graduated this past June with honors and she just finished her 1st semester of college with a 3.87 GPA. We did not waste money... the tutors helped her tremendously. When we started the tutors, she was an average student and she excelled to straight A's.
A lot of times, several sessions one on one will help a student reach the point where they get it and then they can soar!
Best wishes!