Take her back to court and they will make her pay. My SO's ex did the same thing. Said she shouldn't have to pay because she was just so poor. She wrote the stipulated agreement saying they would split the med costs. My SO had custody of the child and he was in intensive counselling. Ex said she shouldn't have to pay because my SO only had the child in counselling to break her financially!!! This is a very messed up child she is talking about here. Judge told her the court rejected that reasoning, and she had an obligation to pay, especially since she was only paying $50/month cs. So she had to pay her share of all back med bills, and he told her to pay 1/2 my SO's attorney's bills. Also she had 30 days to pay whenever she received a new med bill. Boy, was she pissed! Serves her right.
When you pay your share of the bills, only pay your half. Send the medical provider a copy of the decree that explains your ex is liable for the rest. Let them bill her directly. Then take that evidence to court also when she doesn't pay them.