Pic of why your derailure might not be working 100%
I was working on a rear derailure of a Trek Mountain Track mountian bike (1980's?).
I could get the bike into low gear, but had a hard time coaxing it into high gear.
After cleaning and oiling and working the derailure, I finally decided to check the cables.
Ugh! Check this out. This was the loop by the rear stays just near the derailure. Not only was the cable rusted, but it was frayed to the extent I am surprised I could make it move at all inside the cable housings.
Even a little corrosion can cause problems, but in some extreme cases, you get THIS. Here is evidence that you have to check the derailure cables once in awhile.
One of the first Aluminum Rockhoppers to come with front suspension.
A similar thing happened to me, I was putting new housings on my cable, and one strand from the cable got caught in the housings. I forced the cable through, and the one strand bunched up at one spot in the housing.
Low and behold, shifting started to go haywire. Pulled the cable out and recognized my mistake.