In different situations, there can be different reasons for this:
If your hair is dark and you chose a lighter colour, in order to achieve the desired result, you have to bleach your hair 1-2 tones first. Hair Bleach Super Blond ELEA would do this for you.
If your hair has been continually treated with colour, bleach and/or permanent waves, most likely it has become porous. This results in absorption of greater quantity of hair dye and producing more intense colour. You should ask your colourist for a professional help. We suggest that, the next time you reapply this colour to your hair, you cover your new growth first, wait for 20 min, then apply to the rest of the hair. Wash out after another 10 min.