Re-dyeing Hair?

My original hair color is black very black. It was very sick, I had ombre so there is the middle section that was dark brown and the bottoms that were later pretty much bleached but not to where they were blonde blonde it was like a medium blonde. I went to Sally's and got some help from somebody because I wanted to make my entire hair one color a nice dark brown. She gave me a 20 developer with dark brown color, but my black hair didn't take any of it. It still looks black and the bottoms are pretty much dark almost black too. I wanted to go medium dark brown. What should I do now? I obviously don't want to damage anything, but I want to go dark brown not black what do I do next?