Hair doesn't take colour well

I have a client who's hair seems to be resistant to colour (19 yo female,level 6-7 natural, no grey)

The first time I did her hair she had a faded out level 6/7 red on her ends and about 2 inches of natural root.

I used a Demi permanent ( colour wear alfaparf) and mixed 5.1 & 6.0, it literally did next to nothing (I've never had trouble with this colour before)

I went back and recoloured with the same mix in fanola permanent colour& 10 vol and it covered well.. All good.

Second app, I...

Hair doesn't take colour well