You should start a contempt action against your ex.
To protect yourself from a costly contempt action, you should request “income withholding only services,” from your local child support office. You will complete and return an application for this service. The office will keep track of the payments she is making, and if she falls behind, they will initiate the contempt action for you. This is a free service to you, but will cost your ex $15 per month on top of the child support obligation.
If you don’t sign up for this service, you will need to take her to court. To start a contempt action, she must owe at least $500 and be 90 days behind. If you hire an attorney to collect past due child support (arrearages), you are entitled to reasonable attorney’s fees as part of the action. However, this is limited to 30% of the arrearages you are attempting to collect. To receive your attorney’s fees, you must receive a Judgment, and your attorney must file notice of your intention to receive attorney’s fees.
Be aware though if she pays the judgment in full within 20 days, she will not have to pay your attorney’s fees.