Her performance on the 11+ really depends on her individual capability. Try not to compare your child to siblings or friends as she is very unique. If she is not understanding a lot of core concepts in maths and English then that doesn’t mean she will per definition fail. It might be that she simply hasn’t been motivated at school and hasn’t put in the work required to build her problem solving skill set and topic understanding. If you can motivate her to put in the work and work towards her maximum potential it might turn out that this potential is way above the national average and she will easily pass the 11+.
It is impossible for me to tell what your child’s maximum potential is without working with her. But you will be able to test this out. Make sure you start working together with your child way in advance of the exam so that there’s plenty of time to shape her skills. There are numerous examples of students that simply weren’t motivated at school and performed fairly poor at mock exams and placements tests. However, when these students got motivated to prepare for the 11plus exam they quickly started improving their key skill areas. So there's definitely no need to drop the ball by just looking at placement tests.
Why not give the 11+ courses a go? There're for free!