Lately I have been having some trouble with shifting that I haven't been able to fix by adjusting the barrel adjuster or cable tension. My rear cassette has been having trouble shifting into the smaller (harder gears). on my bike this would require loosening the cable to adjust into this position. The problem is no matter how loose I make the cable, I cant seem to get it to shift into the smallest couple gears. When I shift all the way into low gear, I can make it shift by pushing the derailer the last fraction of an inch or so, but it won't seem to go into this position on it's own.

Any guesses? I tried lubricating the derailer mechanism and shift cable but no improvement .