I would check your shift cables at both ends. I had a rear shifting issue on my Tiagra 4700 where downshifts (to the bigger cogs) was fine but upshifts were getting worse and worse. I suspected the cable, but mine were internally routed and not something I wanted to deal with, so I took it to a LBS. The mechanic disconnected the rear shift cable and held it in one hand while shifting up and down with the other. The cable wasn't releasing at all on upshifts to the highest gears, so he popped the hood cover open and showed me that the shift cable had frayed. I gladly paid him to extract the cable and leftover shards plus route a new cable.