permanent color didn't take on roots

I'm looking for any thoughts/advice.

On Sunday I used Redken Hi-Fusion in Red, and a Redken Level 20 developer.  I've used these products before and have had great results, but this time my roots came out much lighter than the rest of my hair.  I have naturally medium-brown hair, and about 30% grey, and I had been coloring my hair red.  I left the dye in for about 1.5 hours (again, which I've done before) and my roots (fresh regrowth) was lightened (maybe even lighter than I'd expect with a Level 20), but it looked like the red color just didn't take.

Any advice?  Should I go back and try to re-do it with a Level 10 developer?  Would I be better off just using a semi-permanent color until I need to do my roots again?  I'd appreciate any suggestions or advice.

Thanks!