Roots lighter than rest of hair - advice?

Hi everyone!

I have a bleach/dye question, but I'll give some quick context. I have quite long hair these days, and don't bleach my entire head of hair every time I touch up my roots - my hair is in ok condition now, though a bit dry, and I'd like to keep the rest of the hair that's not being bleached as healthy as possible. When I bleach my roots and apply SFX Blue Velvet to all of my hair, inevitably the dye on my roots is much lighter/fades much quicker than the rest of my hair. Any tips for this? I'm guessing this might just be the way it has to be, but any advice is appreciated. I try to do all the right things - I don't condition my roots before putting the dye on, then I leave it in for hours, etc. I have a feeling the only way I'd have a really nice even dye job is if I were to bleach my entire head of hair each time, but I just don't want to put the rest of my hair (already bleached in the past) through all that again every 2 months.

Thanks in advance if anyone has any ideas about this!