Child Support Reduction and Jurisdiction

I am divorced, living in the state of Washington, and paying child support to my ex-wife in Colorado. The divorce and child support order are from Colorado. The suppost payments have been current for some years, but there is a sizeable back balance. The payments are made via payroll garnishment, with which I have no problem.
Last year I petitioned the court in Colorado for a reduction in support, and the court granted it. The child support agency in Colorado notified its counterpart in Washington. The Washington child support agency did not reduce the support amount. Instead it said it would consider it is I signed a waiver of the statute of limitations, and filled out financial forms. I asked them which state had jurisdiction, and they said it was Colorado, but since they were the collection agency for Colorado, their decision trumped the Colorado court decision (I am paraphrasing.)
That was last summer. After that I was dealing with a serious health crisis, and I am just now able to return to pursuing this.
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