Yup - made sure that was done. I really think it was a problem with inserting the film holder in front of the ground glass. I'll try again today and see what happens.
What sort of problem? Of course you put the film holder in front of the GG(between GG and camera back), just make sure you pull out the correct darkslide(one closest lens). I dont know what other mistake you could have made other than overexposure.