Quick answer
Use the support-enforcement process; do not try to collect by withholding parenting time.
Do now
Compare the order with the official payment ledger, including payments made directly. Keep bank records and receipts. Contact the responsible support agency or ask the issuing court about enforcement. Identify whether the problem is nonpayment, an incorrect ledger, or an employer withholding issue.
Watch for
Do not assume every enforcement tool applies immediately. A court may need proof, notice, and a hearing. Parenting-time enforcement and financial enforcement should be raised through the appropriate separate requests.
Official sources
NJ Courts — FD modification application packet, NY Courts — Starting a Child Support Case, DC Courts — Parentage and Child Support Branch.