If your child weak on mathematics he or she sometimes have the dyscalculia. These children do not have the ability of logical thinking. They weak not only for calculating but also for geometry. These children cannot do thinking backwards, matching before and after numbers.
You have to prepare an environment to work them alone. Do not advise them for all. Let he/she to think independently. Do not give them all the things. Teach them step by step. Do not teach them even two operations at once. Teach them only one operation at once. Start from the very little thing and go forward. Courage them well. Do not let them to hear "You can't". It causes to draw them back. If the child think "he cannot", you have to dissolve it.
Direct your child to specially trained person for such cases. He/she can understand the child better than ordinary teacher.