Lately I've been having trouble down-shifting (moving to larger cogs) with my Shimano Deore XT 8-sp RD. It's usually in the middle range of gears, from 3 to 6 or so. Sometimes when I move the down-shift lever, the chain will stay on the previous cog and go *tick* *tick* *tick* *tick* *tick* *tick* for a bit and then finally shift to the larger cog. Or sometimes I'll shift back up (chain never moving off the smaller cog), then shift down again and it will go fine.
It doesn't happen all the time. It happens less often than it does not. But when it does it sure is annoying, because I end up losing a LOT of momentum if I'm trying to down shift while going up a hill.
I don't seem to have any problems with the cogs at the end of the range, like 1-2 and 7-8. Any thoughts?