How to modify a support order?

I was in jail and notified the state that I was not working. I finally received a hearing for modification 5 years later and they refused to allow credit from when I notified them of my change in employment. Do I have grounds for appeal? Is it true as the attorney for the support office stated - that credit is not allowed because I was in jail? What about my notification that was sent to them 5 years ago? Any information you can provide is greatly appreciated.