Quick answer
Document the problem and use the enforcement process; avoid escalating the exchange.
Do now
Save the current order and a factual log of dates, scheduled times, messages, and what occurred. Make calm, child-focused communications where safe. Ask about enforcement, clarification, or makeup time.
Watch for
Do not withhold support or physically seize a child. If the issue is an immediate safety threat rather than an ordinary dispute, obtain urgent help and ask about emergency relief.
Official sources
NJ Courts — FD modification application packet, NY Courts — Custody and Visitation, DC Courts — Family Court Self-Help Center.