Many states are trying new ways to make deadbeat parents pay child support. Some states now have laws allowing for the suspension of a deadbeat parent's driver's license. The suspension is lifted when the parent begins to make payments or enters into an agreement to do so.
One state has begun "booting" the automobiles of deadbeat parents. Cars of deadbeat parents are immobilized by baby blue and pink "boots." A sticker is also placed on the windshield of the car stating that the car has been "seized by the sheriff for unpaid child support." Apparently, this has been a very successful method of collection.
Your claim may also be barred by a legal doctrine called "laches." This doctrine is used as a defense when a legal claim becomes stale and prejudicial because a lot of time has passed. There normally must be unreasonable delay in asserting rights which works to unduly prejudice the other party.
The proper way to address this is through a written modification and agreement filed with the court, if your state allows such.