There could be many reasons why your hair is no longer holding the color like it used to, and really it's not that easy to pinpoint.  Are you taking any medication that you didn't before?  Did your diet change?  Is your metabolism the same as it was before?  Sometimes the smallest things can be the cause of the problem.  You should also call the company that makes your hair dye and ask them if they changed the composition of the dye.  It could be that they added something (like a moisturizer for example) to the formula and now it's no longer compatible to your hair.  If there's no apparent reason as to why this is happening, you should try changing hair dye, try another brand.  Purchase a new brand and do a strand test and see how long the color holds.  If the color ends up holding then stick with it.  You can also try a shampoo and conditioner that contains no salt (which helps strip hair dye).  Pureology seems to be a really good brand for that, not only that, but no salt products damage your hair less.  Try:

http://www.pureology.com - www.pureology.com or,

http://www.drugstore.com/search/search.asp?searchtype=1&trx=28198&trxp1=60&srchtree=1&search=pureology&Go.x=7&Go.y=19 - http://www.drugstore.com/search/search.asp?searchtype=1& trx=28198&trxp1=60&srchtree=1&search=pureology&a mp;Go.x=7&Go.y=19  (for reviews).

Good luck and let us know how everything turned out.