How come when i dye my hair the grey will not get dyed?

I have this same problem. Grays are just resistant to color because they are less porous than the hair that has retained it's color. I have found the best hair dye to use to be Clairol's Nice & Easy. And you have to leave it on longer than if your hair isn't gray, I usually leave it on for 45 minutes. It also helps if you can go a little bit darker than your normal shade, not too dark, just a shade or two darker. Good Luck.