Originally Posted by
Al1943
Probably some excess friction somewhere along the cable. This is most often found in the short housing just before the rear derailleur.
Solution is to replace or lubricate the shift cables and housings.
Another possibility is hardened grease or dirt in the shifter.
Solution is to hose out the shifter with WD40.
You could try reducing the tension on the rear shift cable but that may cause other problems.
Al
Just to add it this, many times the stock rear housing loop is too short. Adding a couple centimeters to the rear housing loop can decrease friction.
I find that adjusting the high limit screw so the derailleur rests a couple a mm or two just outside the center of the small cog, then using the derailleurs cable adjuster to add a bit of tension works better than setting the limit screw to the absolute center of the small cog.