Performance on the 11+ really depends on individual capability. Try not to compare your child to siblings or friends as they are very unique. If they are not understanding a lot of core concepts in Maths and English, then that doesn’t mean they will per definition fail. It might be that they simply haven't been motivated at school and haven't put in the work required to build their problem solving skill set and topic understanding. If you can motivate them to put in the work and work towards their maximum potential, it might turn out that this potential is way above the national average and they will easily pass the 11+.

It is impossible for me to tell what your child’s maximum potential is without working with them. 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 their 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 11+ exam they quickly started improving their key skill areas. So, don't let placement tests dishearten you! 

Why not give the 11+ courses a go? They're free!