I recently ran into a problem where I couldn't get anyone to do anything at a leather works. A noble had a mandate on leather backpacks. I had a craftsdwarf with Leatherworking enabled, and a Leatherworks with 3 leather backpacks queued, and no other items. The dwarf had "No Job" and the workshop did not show an "S" or an "A" on any of the backpacks. I reset the workshop profile to permit every dwarf (it was not restricted to begin with, but I restricted it, and then cleared it). I even went so far as to enable Tanning and Craftsworking on the craftsdwarf, but he still just sat around without making any backpacks. I had plenty of leather, and received no error messages about job items or pathing. I also tried enabling Leatherworking (and even Tanning and Craftsworking) on other dwarves that had "No Job" but they wouldn't do anything either.
The only thing I could think of that might have been preventing the work, was my Craftsdwarf Guild noble had recently been injured during a ratman attack. He was resting in his bedroom; a couple craftsdwarves (including the original leatherworker with no job) tended to mill about his room. Do the guild nobles actually hold meetings with the laborers? If so, he may have been interrupted by the attack and subsequent injury.
The mandate expired so I stopped trying for a while, but ultimately I destroyed the leatherworks and rebuilt it, and the noble recovered. I didn't have any trouble thereafter, but I don't know which event fixed it.