Roots Did Not Take Hair Dye?

I applied Garnier Olia black to my roots last night and some of them did not take. They are right at the top of my head. I do this every month to cover gray hairs so I'm pretty sure I did the same process as always. I am not sure if I skipped that area or the dye was just not good for my hair. It was my first time using it. Is it safe to reapply color to that area only or should I just walk around looking like a skunk for the next 3 weeks or so?