chain ridding up and off sprocket problem...

[davidhoare]

We have our drive system all set up (theoretically) however one of our large kit sprockets seems to have a problem. The chain invariably rides up and off the sprocket after a few rotations. We have checked:
- put face to face, the two sprockets look identical - yet only 1 has this problem
- the 2 axles and sprockets (large and small) are all as parallel and/or perpendicular (depending) as can possibly be given our tools and experience...
- the chains we have tried have no obvious defects
- the chain is tight, but not TOO tight (we think)
- nothing 'wobbles' when it spins

Any other ideas? have others had trouble with the kit sprockets? did we get a dud, or can anyone think of a place to start troubleshooting this that we haven't looked at?

Thanks,
David Hoare
Courtice S.S.