My 6 geared(Shimano gears) mountain bike with the grip shifting system shifts really smoothly from the 1st gear to the 6th (biggest cog to the smallest). However when I try to twist the shifter from the highest gear to the lowest, the grip feels tighter and tighter and then gets impossible to shift at the 4th or 3rd gear.
Stuff you may need to know:
Pressing against the derailleur with my thumbs manually changes the gears perfectly.
Removed the cable from the derailluer to check the shifter itself and it pulls and pushes the cable perfectly too.