Dye Job Help. DIY

Ice

I know this isn't something to complain about but I've always dyed my hair at home with help from my sister. The problem is we can nevee get a full even color on my head. Hers will be even but on my hair it'll always come out as a reverse ombre. Even when we first started doing it. My hair is VERY healthy considering it grows very fast and feels soft. We dye it with dirty hair so it has natural oils, we use 2 boxes so there's plenty of dye, we leave it in for an hour (cause brunettes), and a whole bunch of other stuff like starting at the bottom then doing the roots! It's always a reverse ombre color. I mean I like it and I never will use bleach on my hair to whiten it. But I really am wondering why it always comes out as reverse ombre. Anyone else have this issue or have had this happen constantly?