I might be getting confused here but, have you set the High and Low screws correctly? See if you can actually manually push the derailleur out to the 8th sprocket. Maybe the limit has been set too low and so the derailleur can't travel far enough for the pawl in the shifter to engage.
I might be getting confused here but, have you set the High and Low screws correctly? See if you can actually manually push the derailleur out to the 8th sprocket. Maybe the limit has been set too low and so the derailleur can't travel far enough for the pawl in the shifter to engage.
...or something.