You've definitely been on the right track by being so supportive of your daughter and working with her teachers. Rather than expecting her to turn things around all by herself, you can do several things to put her on the road to success in sixth grade.
The most important lesson that your daughter needs to learn is that she now must spend one hour studying at home each day. This works best if it is the same time every day. The hour must be used to review or work ahead if she has no assignments. Just devoting this much time to schoolwork can really improve grades. It's quite possible your daughter has not been studying as much as she really needs to.
Because there's so much more work in sixth grade, your daughter needs to be well organized. Besides using a notebook to list assignments, she needs a calendar to organize her time. Test and report due dates, as well as appointments and activity times, need to be written down. Go over the calendar with her every day until she learns how to plan her time. need to be written down.
Finally, academic troubles often emerge in sixth grade because of weaknesses in basic skills. If necessary, have your daughter work with a tutor or go to a learning center or summer school to improve her skills.