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I will try to best answer your question.
In addition to the UCCJEA I would also advise you to examine the order because if it is a California order there could be a provision that California retains jurisdiction arguably beyond the 6th month.
Court considers factors of significant contacts such as witnesses that would testify as to the best interest of the child such as other family members in the state, friends, teachers, doctors, neighbors, church, exra curciricular activities etc. You basically want to point out all the involvement, activities and connections of all kind the occurred with the child and anyone else in California. However, don't forget the argument of the retained jurisdiction is provided for under the order by CA court.
I hope that this information and advice is helpful and I wish you the best of luck with this argument.