HI, i had exact same issue with my Ultegra 11spd. Eventually decided t check what was happening. all the other gears indexed perfectly. Decided to pull the cable out (also internal routing) at the shifter and thats where i saw the cable started "fraying" inside the shifter and would not let it shift into the smallest gear. replaced and lubed a new cable also cleaned the old existing housing. Its 100% now. hope this helps. ps. make sure there are no small bits of dirt or old cable in the shifter.