Most states don't allow judges to retroactively change child support obligations. This means if a person becomes unable to pay support, the payor may petition the court for a reduction, but, even if the court reduces future payments, it will most likely hold the parent liable for the full amount of support owed at the time. For this reason, if a parent with a child support obligation starts falling behind because the parent's income has decreased or debts have increased, the parent should immediately seek a temporary modification.
For example, let's say Joe has a child support obligation of $300 per month. Joe is laid off, and six months pass before he finds another one with comparable pay. Although Joe could have sought a temporary decrease on the grounds of diminished income, he lets the matter slide and fails to pay any support during the six-month period. Joe's ex-wife later brings Joe into court to collect the $1,800 arrearage. Joe cannot obtain a retroactive ruling excusing him from making the earlier payments.