can't say you're right or wrong. the IRS has its rules for who can claim a qualifying child - generally the custodial parent. the divorce court evidently said alternative years. that means that in the years your ex had the legal right to claim the child you were supposed to furnish him with a signed form 8332 which he was supposed to include with his return. the fact that he did not provide the child support he was supposed to pay you does not mean that the court would say you now have the right to claim the child in all years. you would need to go back to court to have the child custody order changed. we can not answer legal questions.
if he had the court order and the 8332 for 2019, the IRS says he had the right to claim the child despite the fact that he did not pay child support. if 2020 is your year you can claim the child.
It's a quirk in the tax laws that allows in the case of a divorced couple - provided the necessary docs are provided to the noncustodial parent - for that parent to claim one year and the custodial parent to claim the next.
