I have been having an issue on my road bike getting the rear derailleur to reach all of my cogs. I can either not reach my two smallest cogs or two largest cogs depending on the cable tension.
I found a nice set of instructions here: http://www.williambenedict.com/bikes/derailleurs.html and followed through them with the cable disconnected and I was able to set the limit screws so that if I moved the derailleur manually I was able to shift smoothly to every cog. Once I connected the cable I was able to shift to all but the 2 largest cogs so I started adjusting the barrel 1/4 turn at a time. I had to turn the barrel almost all of the way in order to get to the 2nd largest cog(I can't even reach the largest) and with this adjustment I can no longer reach the two smallest cogs.
From what I can tell my derailleur isn't bent at all. Also, I'm not sure if it is much more difficult to shift to a larger cog than it is going back to a lower (I have bar end shifters). I have to pull on it quite hard.
Since I am able to successfully shift to every gear smoothly whilst manually moving the derailleur, I imagine this is an issue with my cable. Would it's age have anything to do with it?