Check the arc of the cable housing at the rear derailler.

Check that the rear derailler cable is not binding in that arc of housing.

Check that all ends of the cable howsing are clean ie there are no ting 'hooks' left from cutting the cable housing.

Check that the ferrules have not deformed. Last year i got a bunch that deformed with sone actually allowing the housing to pull through them.

Check that the cable housing isd CENTERED over the hole in the bar-end shifter that the inner cable leaves through.

Lube the bare inner cable with quality grease. I use MARINE grease for this.

Check that the upper cage of the derailler is clearing the cogs.

Good luck and cheers