Update: A colorist told me my toner mixing bowl was the problem, and she was right
I was mixing toner in a cheap plastic bowl for about a year and kept getting patchy gray spots near the roots. A colorist named Renee at a class in Columbus told me flat out, "Toss that bowl, plastic holds onto old pigment." Switched to glass and my gray coverage evened out in two weeks. Anyone else have a tool quietly wrecking their color before they caught it?