Help with dying hair lighter...

 

My natural hair color is dark brown.

 

I usually dye/streak my own hair with good results. I usually streak my hair with light brown and auburn or just dye it light brown... anyways I had my hair light colored for about two years before I decided to dye it back to dark brown about three or four months ago.

 

So now I want it back to light brown w/auburn streaks, and I'm still using all the same brand of dye, but now only about 2-3 inches from the top of my head will turn light brown, while the rest just becomes somewhat lightly tinted.

 

I'm guessing this is due to the dark brown dye still in my hair... it doesn't look that bad so I'm not panicking (the dark brown did turn a few shades lighter so its not wierd or anything) but I would really like to get my hair back to light brown.. is this even possible with the dark brown dye  in my hair? Basically only my natural hair is getting the right color, while the hair that I've dyed a few times won't change color.