Quick answer
Request an itemized accounting and compare it with the order and your evidence.
Do now
Make a month-by-month table of amounts due, amounts paid, dates, and payment channels. Keep canceled checks, transfer confirmations, and employer withholding records. Ask the agency how to dispute the accounting and whether court review is necessary.
Watch for
A payroll deduction date may differ from the date the agency credits a payment. Keep responding to enforcement notices while the dispute is reviewed; an accounting request alone may not stop enforcement.
Official sources
NJ Courts — FD modification application packet, NY Courts — Starting a Child Support Case, DC Courts — Parentage and Child Support Branch.