The black O-ring which usually ships with it has probably broken and is no longer 'driving' the passive roller.
BC folks say they only use it as a quality control measure to confirm that the passive roller will turn before shipping.
The raised bumps on the rollers are quite sharp and tend to give the O-rings the "death by a thousand cuts" treatment after a few batches--eventually the ring snaps and ends up in your grist
If the mill gap is set properly, then the grains will drive the passive roller during normal use.
You can buy more o-rings in the plumbing dept at HD or Lowes (plumbing / sink fixtures area) if you want, but they are not needed. I recommend not getting them because I hate looking for the rubber pieces in the grist if one does snap.
If the passive roller isn't turning at all, then I'd disassemble the mill base and find out if there's any flour that has gotten in between the roller/axle and the housing.