working from life or a photo? if it is a photo turn it upside down. When i was in life drawing once i had an odd angle (i always picked the hardest foreshortened pose on purpose) and i couldn't get it no matter how hard i tried. My teacher came over with a piece of clear plastic that we held up to the model and outlined with a magic marker so we could look at the outline only..ended up it was just such a weird angle. From life is a lot more tricky, sometimes you got to just keep penciling it in and erasing it out until you get it even by accident..been there, done that! Sometimes a few free quick lines will be IT.