Joined: Jul 2015
Posts: 7,498
Likes: 3,301
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
possible bent/kinked Cable or Cable Housing.
possible Frayed Cable in the shifter area.
replace cable, noting any high drag while inserting new cable.
replace housing that causes any noted drag.
Tip: replace Brifter cables Regularly... once a year if you ride a lot, every Two years otherwise...
some of the Most Epic struggles reported here, and that i've experienced, are Removing the broken cable end from the Shimano brifters... it is NOT fun.
it ranks worse than getting a Shimano shift cable out of an old Campy downtube shift lever......
your symptoms point to a possibly Frayed shift Cable.
Replace Left and Right cables as PM, to avoid the Epic Struggle.