On my 2000 series Janus door the axle is a dead axle (does not move). The drum is what spins and is on bearings.
On the reduced chain drive there is a ring gear bolted to the drum.
There is a metal gear box mounted to the on the end of the angle bracket. This metal box contains to chain roller which turns a small gear that engages the large ring gear on the drum.
There is probably a pin , shear pin maybe idk.
but if the chain is rolling then it seems it would have to be the small gear is not turning.
I would also check the alignment of the gear box and the gears where they mesh together.
If misaligned it will not engage the ring gear.
On mine the small gear box is not really adjustable though.
I will say that if the angle bracket is bent away from the drum the gear box will move away from the ring gear and not make contact.
Hard to explain. Can you take pictures?