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You can still file your claim for dependenciy even though ex husband has claimed same child. What will happen is the IRS will issue a refund based upon deduction for both of you. They will then realize that two the exemptions have been given out. They will then go back and determine who should have properly filed the dependency exemption. It normally goes to the custodial parent unless there is an agreement in writing that says otherwise. If you were entitled to it you will be allowed to keep your deduction and ex husband will have to refund money back to IRS.
In the event that ex husband is not following the terms of the custody order you can file a petition for contempt in order to force compliance. If the provisions he is not following are not in the custody order you will have to modify to add and than at that point you would then be able to enforce the provisions.
I hope this information is helpful and I wish you the best of luck.