From advice given to me by my own hairdresser regarding colouring problems caused by my previous hairdresser, I am guessing that you are getting what is called colour build up. As you dye and dye your hair, you are probably putting fresh dye onto already coloured hair, in some cases it can turn the previously dyed hair almost black as it has so much colour added and added to it.
When I have my hair coloured, my current hairdresser carefully does the roots, only putting it on the freshly grown hair, avoiding the hair that already contains dye from the previous time. If they want to refresh the whole head, once the roots have taken, they then comb out the dye over rest of the hair, but only leave this on for a very short time to avoid any new colour build up.
They say that over coloured hair has a very flat look to it (ie no shine) and they took a lot of time and effort with mine with some very careful colour correcting, to get rid of the bands of colour build up that the previous salon had done to my hair.
If this is the case with your hair, and you do not trust a hairdresser, I would try karenmac60's advice and see if you can buy a colour remover, but I would advise being very careful as you do not want to make it worse!
Hope this has helped.
Sue