Hair Wont Hold Color

What kind of shampoo are you using?? Is it for color treated hair??
One of the things that I have noticed from some of my clients is that when they buy a "volumizing" shampoo, that sometimes will fade the color because the only way to volumize hair is to push open the cuticle and when you do this you will release color molecules thus fading your hair. 
Also you might want to try to post fill your hair.  What I mean by this is to apply a non peroxide (semi permanent) clear formula on the hair when you are finished applying your color and have finished shampooing, (do the post fill prior to applying conditioner).  Goldwell makes a semi permanent clear that I use all of the time to post fill.  This helps me maintain holding power much better

OK grey hair is pigmented hair. Many people assume or believe because hair is grey that it has NO pigment. This is NOT true. Grey hair has pigment.

Are you using Ash Blonde to try and color up your grey? I'm not sure what product line you are using but many times the lightest level or color for example Level 10 blonde from Goldwell (Their level system goes from 2-10, others sometimes go from 1-12) won't cover grey. So you need to drop down a level to a Lvl 9 to get the grey coverage you want.

 

Try 'Lock In Blonde' from Sally's. It worked wonders on mine that had absolutely NO pigment left.

Apply a protein filler prior to dyeing. This should fill "porous" hair that
many blondes seem to have. Also, don't wash hair two days before and
two days after dyeing. Especially after, because it will give the dye time
to penetrate the follicle. Hm, does anyone have any tips on which color
filler to use with which shade dyes?