Mine does that a lot. Sometimes it's just over over colored and it won't take anymore.

You're really only supposed to permanent color your roots, and touch up the length with a demi.

And I've read that color won't lift color, so unless you're coloring with a darker color, it just might not work.

Two things I do in this situation is to just wait. Apparently the porosity if my hair at that time is not accepting the color, and/or, use a protein filler. I've just found you can get these at Sally's, and used before coloring, it can help your hair grab the color. Follow the directions, and then just put the color right over the protein filler.