Are you sure you harvested the nettles, using Agriculture skill, instead of just picking them up? If you just pick them up, you get leaves, but if you use Agriculture and Harvest them, you should get nettles that you can then turn into yarn using the Extract fibers command in the Textilecraft skill menu (or do the full process if you have the time for it). If you only ever get leaves, no matter what you do, it sounds like a bug.