Windows partition doesn't show in Bootcamp

As said in the title, when I load up the computer and keep the option button pressed down, only the macintosh disk drive shows on the computer. In the bootcamp menu thing and the start-up disk option, you can however see the partition and choose it to star-up from.
HAs anyone got any suggestion as to why the disk is not visible upon start-up? In the apple store they told me to re-install windows...which I have done 3 times now!!! still doesn't show, cheers for any advice.