HELP: Color Doesn`t Take

I`m having a problem with my home coloring. About two and a half months ago, I colored my few greys with a semi-permanent, good-for-24-shampoos hair color (Natural Instincts). Funny thing is that it never washed out. Because I have visible roots now, I decided to try a permanent hair color (Hydrience Reds) this evening. The result: it colored the virgin growth of my very dark black, Asian hair beautifully. However, the previously colored areas didn`t seem to take the color.

Two questions:

1. Why does this happen?
2. How do I fix or avoid this phenomenon when I color again?

Thanks!