My daughter had the exact problem when she first started taking Concerta 3 yrs ago at the age of 7, except that she'd go 6 weeks before her behavior became a problem again. After upping her dosage to the most it could be upped due to her weight, her doc then tried her on Adderall. The same thing happened, after 6 weeks her problems would start back up. Her body kept adjusting to the medicine. So, her doc came up with the idea of having her take one med on the weekdays and the other med on the weekends. Now, she'll go about 6- 8 months before the meds stop working. When that happens, we switch the meds - ex: if she had been taking Adderall on the wkdays and Concerta on the wkends, then she would do Concerta on the wkdays and Adderall on the wkends. This has been working like a charm for 2 yrs now. Ask your doc about this protocol but try the Adderall first because sometimes Adderall works better for a child than Concerta and vice versa.
Hang in there. We understand what you're going through!