GARNIER NUTRISSE HAIR COLOUR CHANGES

I have used Garnier Nutrisse and absolutely loved the color and condition it left my hair in, but for some reason the last two applications the color has changed and did not take on my hair despite leaving it on my roots for 45 minutes. (I use the 4 1/2 Medium dark brown). The color on the last two boxes I used came out noticably lighter than past applications. Disappointed with the results and afraid to buy again in case I experience the same bad results.