My first guess: you probably forgot to calculate the bellows factor. I estimate or actually measure the magnification ratio by putting a ruler in the scene at the subject position and comparing it to the width of the groundglass, and then I have a little table on the back of each camera that gives me the exposure correction based on the magnification ratio and reminds me to include it.

My second guess: you remembered the bellows factor and that put you into reciprocity territory, and you forgot to include the reciprocity factor. It's always something else, isn't it?