You cannot petition "the enforcement group" because it is the court that drives enforcement (support collections). Instead, all adjustments always relate back to the time of filing because the Bradley Amendment prohibits adjustments by the court outside of the petition.
The modification relates back to the date you filed the petition, if it is that same petition that has been at issue for the past 2 years. Since 1986, you cannot go back further based upon federal law prohibiting it.
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The support adjustment is retroactive to the date that the petition seeking the adjustment was filed. The support order should reflect the retroactive date and the support collection unit will make the necessary adjustment to your account.