What can be done when an ex doesn't pay on a mortgage that they are deemed solely responsible for?

It depends on the terms of your agreement or court decision. But if she is responsible and is not fulfilling the responsibility, you can file what is called a “motion to enforce litigant rights.” This is a court process in which the court can order her to comply with the agreement or decision to bring the mortgage current or, within a specific limited amount of time, place the house for sale.

The court is also empowered to order any other reasonable relief as well as the payment of the legal fees you incur in seeking this enforcement.