It's difficult to either lubricate the cable or judge whether there's excess friction without unshipping the housing end from the stop or detaching the cable end so that you can check the upper run of cable by pulling it through from the lever end. You can then lube the cable with oil (not grease) as you feed it back in. If the derailleur is clean and lubed the cable or shifter would be the only things that would cause the derailleur to stop too soon when shifting inward.